The 22-year Slovak gave a command performance on his debut in a full Tour stage by outsprinting Swiss rider Fabian Cancellara, who mounted a spirited and successful defence of his yellow jersey over a hilly ride in eastern Belgium in Stage 1.
The standings among the top contenders to win the three-week race didn't change much after the 198-kilometre loop from Liege to suburban Seraing featuring five low-grade climbs. Bradley Wiggins of Britain and defending champion Cadel Evans trailed close behind in a splintered pack.
Overall, Wiggins is second behind Cancellara, seven seconds back, and Evans is another 10 seconds slower in eighth.
Sagan, who won five of eight stages in this year's Tour of California among the 13 stage wins he has this year, placed his hands on his shoulders after edging out Cancellara and Norway's Edvald Boasson Hagen in third. It culminated a tricky uphill patch with cobblestones right before the finish.
The promising Slovak becomes the youngest rider to win a Tour stage since Armstrong won his first of his 22 career Tour stage victories at age 21 — in Stage 8 in 1993. The youngest of all time is Italy's Fabio Battesini, who was 19 when he won one in the 1931 Tour.
Sagan landed another spot in the history books: No other Tour debutant has won the first Tour road stage since Fabio Baldato of Italy 17 years ago, according to Infostrada, a Dutch sports information service. David Zabriskie of the United States won the opening time-trial in his first Tour in 2005, but that wasn't a full road stage.
Asked about whether he has the potential to be the next Armstrong, Sagan cautioned that such talk was a bit premature, "I would like to be, but I'm so young it's impossible to know what the future will be."
"If that could be true, it would be great!" said the Liquigas-Cannondale rider with a nervous laugh.
He rose to 23th place overall, 24 seconds back of Cancellara, after entering the day in 56th place — 3:49 behind — after the prologue that Sagan said wasn't suited to his talents.
To have a chance to achieve Armstrong's legendary stature, Sagan will have to prove that he's a complete rider who can excel in time-trials and the mountains — not just a "puncher" who can burst ahead late on flatter stages.
He'll get that chance in two time-trials ahead and as the race veers to the Alps in Week Two and down into the Pyrenees mountains before the July 22 finish on Paris' Champs-Elysees.
Cancellara, the 31-year-old veteran who won Saturday's prologue, offered racing panache with an aggressive final attack — an unusual move because the bearer of the yellow jersey generally spends more effort trying to defend it than going on the offensive himself.
"We got absolutely no help from other teams, so I said the best defence is to attack," he said, referring to his RadioShack squad. "I saw the last turn with a little stretch of cobbles and I said 'here, full gas.'"
"I'm not the kind of rider that gives up with 500 metres left," said the Swiss veteran. "That's not me. If I try something I go all the way — and if I finish second, that's just how it goes."
Evans, the Australian leader of the BMC team, rose to eighth place overall from 13th as others slipped back in crash traffic or because of the hilly final climb that split the pack during the last three kilometres.
That was when the stage turned into a three-man race. Sagan hugged the wheel of Cancellara, who was doing the hard work of leading into the wind, then whipped around him with less than 150 metres before the finish to win in four hours 58 minutes 19 seconds.
"I am really, really happy," Sagan said. "I was the only one who could follow (Cancellara), I was tight behind him. I was just happy to stay on his wheel."
At least two crashes marred Sunday's stage amid escalating tensions within the pack near the finish, where roadside crowds drew in to get a glimpse of the whirring bicycles.
High-profile riders including Spain's Luis Leon Sanchez and Michael Rogers of Australia went down in one late spill, but got back up. The bad luck continued to stick with Germany's Tony Martin, who went down in a crash early Sunday before recovering. The world time-trial champion popped a flat and lost time in the prologue the day before.
At one point, with his team leader Evans riding in his wake, Marcus Burghardt of Germany caused his bike to jump to avoid a plastic bottle in a downhill patch about 17 kilometres before the finish.
Six breakaway riders jumped out of the pack after the first kilometre, and held onto a lead until less than 10 kilometres to go — when the speeding pack overtook all the escapees.
Evans said a conservative start by the pack set the stage for a "hectic" finish during the final climb. He took a chance with a burst of speed with about two kilometres to go, after team scouting report led him to believe he could nibble some seconds from his main rivals. But instead, he ran into a headwind and the pack was able to keep pace.
"The first stages of the Tour: Everyone's so keen to get going, and everyone's so nervous," Evans said.
Monday's second stage takes the pack on a mostly flat 207.5-kilometre jaunt slicing west across Belgium from Vise to Tournai, which could favour a sprint finish.