The once-embattled quarterback stoically marched San Diego downfield against Kansas City on Sunday, and then found his unheralded wide receiver with a 26-yard scoring strike with 24 seconds remaining that gave San Diego a 41-38 victory and ended its three-game losing streak.
"It's one of those you'll never forget, that's for sure," said Rivers, who threw for 392 yards and three touchdowns against a Kansas City defence that had been among the NFL's best.
"It's kind of what our season's been about," Rivers said. "Can you drive and score at the end?"
The touchdown pass to Ajirotutu answered one that Alex Smith had thrown to Dwayne Bowe with 1:22 left in the game — one that had seemingly given the Chiefs (9-2) the victory.
It turned out there'd be more lead change in a game that had eight of them.
"Philip said, 'Fly on the boundary,'" Ajirotutu said, "so I knew that's a little code word that he usually says that the ball's coming to you."
Smith wound up throwing for 292 yards and three touchdowns for the Chiefs, who lost their second straight after a 9-0 start. They also lost top pass rushers Tamba Hali and Justin Houston to injuries, and now face the prospect of playing the Broncos next week without them.
Hali has a sprained right ankle and Houston a sprained left elbow, and Chiefs coach Andy Reid said that both of them are due for MRI exams on Monday.
"I'm not making excuses," Reid said. "The next guy has to come in and we expect them to step up and make plays. There's no excuses."
San Diego (5-6) finished with 491 yards of offence against a Chiefs defence that had allowed more than 17 points just once: last week's 27-17 defeat in Denver.
Danny Woodhead had touchdowns rushing and receiving as he picked up the slack for Chargers running back Ryan Mathews, who left with a hamstring injury. Ladarius Green had a 60-yard touchdown reception in the fourth quarter, while Keenan Allen had nine catches for 124 yards.
Of course, it was Rivers who made the entire offence sing.
"I'll tell you right now, we've got a great quarterback," Chargers coach Mike McCoy said. "There was no doubt in my mind at the end of the game he was going out and scoring."
Jamaal Charles added 115 yards rushing and two touchdowns for the Chiefs, while Donnie Avery had four catches for 91 yards and a score in their best offensive game of the year.
"I mean, you'd like to take positives away form games," Smith said, "but right now it wasn't enough. We didn't do enough as a team to come out on top."
The game turned into a back-and-forth nail-biter in the second half.
San Diego pulled ahead 17-14 when a 54-yard pass to Eddie Royal set up a 1-yard TD run by Mathews. But the Chargers helped the Chiefs take the lead right back with three pass interference penalties that gave them the ball at the San Diego 1. Charles' second touchdown made it 21-17.
The Chargers' struggling defensive backfield got one back on the Chiefs' next series. Shareece Wright, who had one of those pass interference penalties, batted a pass to Marcus Gilchrist, who had one of the others. The interception set up Woodhead's 3-yard touchdown run.
Kansas City retook the lead at 28-24 on Smith's short pass to Anthony Fasano, but after the teams traded chip-shot field goals, the Chargers took it right back.
Rivers hit Green on a quick slant, and the tight end ran 60 yards for his first career touchdown. It gave the Chargers a 34-31 lead with 7:50 left.
The Chiefs answered the call, only for the Chargers to trump them in the end.
"That's what the game's all about," McCoy said. "When the game's on the line, who's going to come up and make the big play? Who's going to make the big stop?"
Notes: The Chargers have won 10 of their last 12 against Kansas City. ... The Chargers' Antonio Gates had three catches for 21 yards, making him the fourth TE in NFL history with 700 catches and 9,000 yards. ... Smith dropped to 0-3 against San Diego as a starting QB. ... Wright left briefly with an undisclosed injury but returned to the game. .. The Chiefs played without starting RG Jon Asamoah (calf, shoulder) and RT Eric Fisher (shoulder).
Online: http://pro32.ap.org and http://twitter.com/AP_NFL