Tomas Berdych became the most high-profile casualty of the French Open to date as he fell to French qualifier Stephane Robert in five sets.
The Czech eighth seed, who beat Andy Murray en route to the semi-finals in 2010, raced into a two-set lead.
But 31-year-old Robert, ranked 140th, came back to win 3-6 3-6 6-2 6-2 9-7 in three hours and 23 minutes.
Elsewhere, Gael Monfils came back from a set down to beat Bjorn Phau in four sets and Richard Gasquet also won.
There were victories too for Juan Martin del Potro, Mikhail Youzhny, Viktor Troicki and Thomaz Bellucci.
It was, however, Robert's stunning triumph over 25-year-old Berdych that stole the headlines.
The right-hander had never previously won a match at Roland Garros and this was only his second win in eight Grand Slam appearances.
It was also only his second win of the season on the ATP World Tour and earned him a second-round meeting with Italy's Fabio Fognini, who overcame Denis Istomin 6-4 6-2 6-2.
"I gave it all today," said an ecstatic Robert, who hit 62 winners to his opponent's 40. "I fought for my life.
"In the third set, he started to give me some points. Winning that set gave me confidence and I became more aggressive on the return."
Berdych committed fewer unforced errors than Robert - 33 to 42 - but squandered a match point and admitted: "That's tennis.
"Maybe it's too early to analyse how was that or what was happening on the court. That's what can happen in the long matches. Sadly it happened to me today."
One of the most eagerly-awaited matches of the first round took place on Court One as 2009 US Open champion Del Potro, seeded 25th, took on 6ft 10ins Croatian Ivo Karlovic.
Both players are back on the tour after injury problems in 2010 and it was Argentine Del Potro who came through in four sets, winning 6-7 (7-9) 6-3 7-5 6-4.
The 22-year-old, who remains on course to meet Novak Djokovic in round three, stated: "It was a really tough match.
"It's difficult to play against Karlovic because you don't have many chances to break his serve, but I played a good match. I was focused from beginning to end, and I got through it."
After a shaky start, ninth seed Monfils came good to secure a 4-6 6-3 7-5 6-0 win against Phau of Germany.
Russian 12th seed Youzhny was a straight-sets winner over Japan's Go Soeda, as was 15th seed Troicki of Serbia against unseeded German Julian Reister.
France's Gasquet, the 13th seed had few problems as he defeated Radek Stepanek of the Czech Republic 7-5 6-3 6-0.
Bellucci, the 23rd seed from Brazil and something of a dark horse after his run to the semi-finals in Madrid, was not quite as convincing but saw off Andrey Golubev 6-4 6-4 6-7 (4-7) 7-6 (7-5) to book his place in round two.
Meanwhile, former world number two Tommy Haas, playing his first match in 15 months after coming out of retirement, lost 6-4 4-6 7-6 (7-1) 6-4 to Turkish lucky loser Marsel Ilhan.
Nikolay Davydenko the Russian 28th seed beat Denis Gremelmayr of Germany 7-6 (7-2) 6-1 6-3 and Janko Tipsarevic, the 29th seed from Serbia beat Brian Dabul of Argentina 7-6 (7-1) 6-1 6-0.