Stan Wawrinka became the first major casualty at this year's French Open with a wayward performance against Spain's Guillermo Garcia-Lopez.
The Swiss third seed and Australian Open champion made 62 unforced errors in a 6-4 5-7 6-2 6-0 defeat.
Wawrinka, 29, had been trying to become the first man since Jim Courier in 1992 to win both the Australian and French Open in the same year.
Garcia-Lopez, ranked 41st, now faces France's Adrian Mannarino in round two.
Even so, Wawrinka had a 4-2 record from their previous encounters and had won his first Masters 1000 title on the clay of Monte Carlo only last month.
"The match wasn't good at all," said Wawrinka. "I was trying to find my game, trying to be aggressive, trying to find anything, and I didn't.
"I don't have all the answers for why I didn't play that good. I feel some I need to take a few days off to see exactly what's happened."
He looked anything but a reigning Grand Slam champion on Court Philippe Chatrier, however, and appeared uncomfortable from the early stages after play did not get under way until approaching 6pm local time.
An early break of serve was quickly snuffed out by Garcia-Lopez, who took the opening set in 36 minutes, and after getting back on level terms in the second, Wawrinka's challenge evaporated.
The Swiss, who was in the same quarter of the draw as Andy Murray, could barely keep the ball in court as he won just two games in the final two sets.
"I was feeling OK," Wawrinka added. "I was practising well. I was feeling OK with the pressure, for sure. I always put a lot of pressure on myself, but before the match and during the match it was not really about only the pressure.
"I think it's just a different story. Now it's a different picture for my career.
"I need to put the puzzle back together, but differently than in the past, because now, after winning a Grand Slam, a Masters 1000, being number three in the world, everything is different, and I still didn't find all the pieces."
Garcia-Lopez added: "I had my foot on the accelerator 80%, and when you play this kind of player they try to destabilise you, but he did not manage it because I felt very strong today.
"And I think at the end of the day what made him lose is that he was not so strong mentally, whereas I felt very comfortable."