An irate UFC president Dana White reacted to the decision for St-Pierre in disbelief, even calling for the Nevada governor to launch an investigation into the Nevada State Athletic Commission. He wondered out loud how anyone could score the fight for the champion.
Hendricks fell to his knees as the decision was announced 48-47, 47-48, 48-47 for the Canadian, who took a beating en route to the win before a crowd of 14,856.
"Without a doubt, my toughest fight," said a battered St-Pierre.
"I couldn't see out of one of my eyes," he added. "He really messed me up."
St-Pierre (25-2) said he needed to take some time to mull over his future and made a point of thanking the UFC before he left the cage.
"I have to step away for a little bit," said the 32-year-old from Montreal.
That drew fire from White, who said Hendricks deserved an immediate rematch.
Judges Sal D'Amato and Tony Weeks scored rounds one, three and five for St-Pierre. Glenn Trowbridge scored the first, second and fourth for Hendricks.
"I am the champion," Hendricks told the post-fight news conference.
"The judges ripped my heart out tonight," he added.
St-Pierre was hurting elsewhere. He went straight from the cage to the hospital.
His face covered with red welts, GSP arrived midway through the news conference after being stitched up.
He said he left his soul in the octagon against Hendricks.
St-Pierre declined to detail the personal issues bothering him other than they were family matters, he couldn't sleep at night and needed a break.
After speaking to St-Pierre privately after the news conference, a calmer White said the champion was not retiring and would be back in the rotation once his personal problem was sorted. He did not detail the issue in question but said it was not as bad as GSP thinks.
The two fighters collected an extra US$50,000 for fight of the night. St. Pierre made a number with a few more zeros at the end, given his position as the UFC's biggest pay-per-view draw.
Hendricks (15-2) did what other challengers could not do, take GSP out of his game. He blunted St-Pierre's wrestling attack for much of the night and hurt him with strikes and knees.
The challenger smiled and held his hands up in the air when the final bell rang. St-Pierre did too, but his face did not look like that of a winner.
"I thought I clearly won the fight," said Hendricks, who called St-Pierre a great competitor.
Hendricks started strongly but St-Pierre absorbed the onslaught and held strong, biding his time for the championship rounds. The fight was mainly contested on the feet as the two cancelled out their considerable wrestling skills.
St-Pierre's face showed the toll of the fight while Hendricks, despite absorbing punches, showed few marks.
The card at the MGM Grand Garden Arena celebrated the UFC's 20-year anniversary. Brazilian jiu-jitsu ace Royce Gracie won UFC 1, an eight-man elimination tournament, on Nov. 12, 1993, at McNichols Sports Arena in Denver.
Hendricks, an Oklahoma native who now makes his home in Dallas, walked out to country music. St-Pierre, wearing a gi, came out to French rap. In the stands, giveaway red and blue bracelets flashed during the introductions.
GSP was close to a 2-1 favourite against Hendricks, a former NCAA wrestling champion with one-punch knockout power. The crowd was solidly behind the champion.
Hendricks smiled as the two fighters touched gloves before the first round.
The crowd roared as St-Pierre landed a takedown within 20 seconds. But Hendricks quickly got up.
At the 40-second mark of the first round, St-Pierre surpassed B.J. Penn's record of five hours three minutes and 51 seconds for career fight time in the UFC.
St-Pierre pushed for another takedown, eating some punches and elbows in the process. The crowd chanted "GSP, GSP." But Hendricks took the champ down briefly as they clinched at the fence and St-Pierre was cut by the eye.
Hendricks repeatedly kneed St-Pierre in the clinch with the champion using kicks to keep him away when they separated. It was a good round for the challenger.
Between rounds, St-Pierre's corner told him to box and wrestle, not just box.
Hendricks scored with uppercuts but GSP got a reprieve when Hendricks lost his mouthpiece. At the fence, Hendricks used knees to soften up the champion, who fought back with his jab and kicks.
Hendricks seemed to be slowing slightly as the round ended. But St-Pierre's face showed damage.
In the third, St-Pierre was busy with strikes as the fight remained standing. Hendricks was not as active but scored a late takedown, only to see GSP fight his way back to his feet.
St-Pierre jabbed and kicked in the fourth to a thundering chant of "GSP, GSP." The champion found himself on his back, seemingly from a slip, allowing Hendricks to score damage from above. Hendricks surprisingly let a bloodied GSP get back to his feet so they could resume punching.
St-Pierre came out desperate in the fifth, taking the fight to Hendricks. He scored with strikes and then took him down. But the challenger did not break.
St-Pierre continues to rewrite the UFC record book.
It was a record 19th win in the UFC for St-Pierre, moving him past Hall of Famer Matt Hughes at 18. It also extended his string of victories to 12, the longest current run in the UFC.
St-Pierre has not lost since his first title defence in April 7, 2007, when 10-1 underdog Matt (The Terror) Serra knocked him out three minutes 25 seconds into the first round at UFC 69 in Houston. Three fights later, at UFC 83 in April 2008, St-Pierre avenged the loss and won his title back
The victory moved GSP past former middleweight champion Anderson Silva for most wins in UFC title bouts at 12.
Saturday's win was GSP's ninth successful title defence and ninth straight title defence, one behind Silva in each category. And it was his 14th championship fight, one behind Randy Couture.
St-Pierre came into the bout holding the UFC mark for most championship rounds fought (47).
Earlier, veteran (Ruthless) Robbie Lawler won a split decision over Canadian welterweight Rory (Ares) MacDonald in a wild back-and-forth fight that was judged 29-28, 28-29, 29-28 for the underdog Lawler.
MacDonald (15-2) drew criticism from White for his performance last time out in a win over Jake Ellenberger. But he got plenty of love Saturday, at least early on from the crowd.
He walked out to cheers and Rihanna's "We Found Love." The ovation grew as he was introduced in the cage and there were chants of "Rory, Rory" within the first minute.
The chant changed to "Robbie, Robbie" in the third round as the 31-year-old Lawler (22-9 with one no contest) knocked the Canadian down and punished him. A tired MacDonald managed a late takedown and some ground and pound of his own, finishing the fight with a flurry of blows.
The 24-year-old MacDonald, who came into the fight a 4-1 favourite, was ranked third among welterweight contenders while Lawler was No. 10.
A native of Kelowna, B.C., who trains alongside GSP in Montreal, MacDonald has said he won't fight St-Pierre. Saturday's loss will take care of that, at least for the time being.
Lawler was aggressive in a close first round with MacDonald looking to use movement, kicks and jabs to blunt his opponent's attack or spin out of range. Lawler kept coming in the second round but seemed to be tiring. And when he slowed, MacDonald countered. He also scored a takedown to take the round.
In the co-main event, former light-heavyweight champion Rashad Evans pounded out a first-round TKO over Chael Sonnen in a fight between friends and co-workers as Fox TV analysts.
After they clinched at the fence, Evans (24-3-1) managed to take Sonnen (29-14-1) down and then improved his position, taking his back and hammering away until referee Herb Dean stepped in at 4:05.
Earlier, Tyron Woodley knocked out veteran welterweight Josh Koscheck at 4:38 of the first round.
Koscheck (19-8) was driven backwards by an early right. And referee Herb Dean hovered over the fighters as Woodley battered Koscheck on the ground later in the first round. But the veteran hung on — briefly.
Woodley (12-2) ended it brutally, crumpling Koscheck with a right to the chin and then hitting him with another on the way down. It earned him $50,000 for KO of the night.
Russian flyweight Ali (Puncher) Bagautinov won his 10th straight via a 29-28, 29-8, 30-27 decision over seventh-ranked Tim Elliott (10-4-1). Bagautinov (12-2) landed a lot of punches but had to fight off a second-round guillotine from the unorthodox and durable Elliott, who was not afraid to absorb several strikes to land one of his own.
The undercard featured more perspiration than inspiration although lightweight Donald (Cowboy) Cerrone and welterweight Rick (The Horror) Story were on their game.
Cerrone (21-6 with one no contest) used his striking superiority to batter Evan Dunham (14-5) before submitting him in the second round. Cerrone, ranked No. 10 among 155-pound contenders, stuffed Dunham's takedowns and then scored one of his own before locking in a triangle choke at three minutes 49 seconds of the second round.
Both men needed a victory, having each lost two of their last three bouts. Cerrone picked up $50,000 for submission of the night.
Story (16-8) dominated with punishing strikes and leg kicks en route to a unanimous 30-27 decision over Brian (Bad Boy) Ebersole (50-16-1 with one no contest). Ebersole, his chest hair shaved into an arrow pointing upwards, made as if Story wasn't hurting him but clearly took a lot of punishment.
Ebersole later said he has been fighting through back pain in recent bouts.
Brazilian middleweight Thales Leites used his grappling skills to have his way with Ed (Short Fuse) Herman (21-9 with one no contest) on the ground and win a unanimous 30-27 decision in a gruelling bout.
Leites (22-4) is now 2-0 in his second stint with the UFC, faring poorly in a 2009 title shot to Anderson Silva at UFC 97 in Montreal in his first go-round.
Bantamweight Sergio (The Phenom) Pettis (10-0), the younger brother of lightweight champion Anthony (Showtime) Pettis, won his UFC debut via a 30-27, 30-27, 29-28 decision over a hard-nosed Will Campuzano (13-5).
Welterweight Jason (The Kansas City Bandit) High (19-4) won a unanimous 29-28 decision over UFC newcomer Anthony (The Recipe) Lapsley (25-6 with two no contests) in a fight contested mainly on the ground.
Mexican bantamweight Erik (Goyito) Perez looked impressive in earning a unanimous 30-27 decision over Edwin (El Feroz) Figueroa in a spirited scrap.
For Perez (14-5), it was a return to the win column after a loss to Takeya Mizugaki in August snapped a run of eight straight victories. Figueroa (9-4), who has now lost three straight, earned points for making it through the three rounds. He said later his knee "popped out" in the second round.
Light-heavyweight Gian Villante (11-4) posted his first UFC win with a TKO over (Donnybrook) Cody Donovan at 1:22 of the first round to open the card.