Twice runner-up Dominic Thiem suffered a shock loss in the French Open first round as Pablo Andujar produced a stunning comeback against the world number four.
Spain's Andujar trailed Austria's US Open winner Thiem by two sets but triumphed 4-6 5-7 6-3 6-4 6-4 in a thrilling match at Roland Garros.
German world number six Alexander Zverev had to fight back from two sets down to beat compatriot Oscar Otte.
Zverev, 24, won 3-6 3-6 6-2 6-2 6-0.
Elsewhere in the men's singles draw on Sunday, Greek fifth seed Stefanos Tsitsipas beat France's Jeremy Chardy 7-6 (8-6) 6-3 6-1 to move into the second round, where he will face either American Sebastian Korda or Pedro Martinez of Spain.
And British number one Dan Evans fell to a 1-6 6-3 6-3 6-4 defeat by Serbia's Miomir Kecmanovic.
'Everything was not the real me'
Thiem's defeat leaves the bottom half of the men's singles draw especially open because the 'big three' of Novak Djokovic, Rafael Nadal and Roger Federer are all in the other half.
Having lost to Nadal in the 2018 and 2019 finals in Paris, Thiem earned his first major title at the US Open last September but took two months away from the tour earlier this year to have a "small reset" because of the physical and emotional impact of winning a Grand Slam.
Before his opener at the French Open he had won only five matches in the past three months.
"I was not struggling at all with my motivation, but the game was just not there," said Thiem.
"All the shots are missing power. They are not accurate enough. I'm moving not well enough. Everything in my game there are some percentages missing.
"Everything was just not the real me, or the version of me who is able to play for big titles."
World number 68 Andujar, 35, turned professional in 2003 and, although he beat Roger Federer in Geneva earlier in May, this was his first victory over a player ranked in the ATP's top five.
Having broken Thiem's serve in the eighth game of the third set to end the Austrian's hopes of sealing a comfortable win, Andujar kickstarted a memorable comeback by racing into a 4-1 lead in the fourth.
Neither player could hold serve at the start of the deciding set, but Andujar consolidated another break to take a 4-2 lead and a stunned Thiem could not find a response.
"It's a very special moment because I'm 35 and I don't know how long I'm going to play," said Andujar, who will play Radu Albot or Federico Delbonis in the second round.
"It was very difficult, in the third set he wasn't quite as strong. I didn't think I was in my best shape, but the belief was the key to the win today."
Zverev avoids opening-round exit
Zverev, who won his second Madrid Open title earlier this month before losing to Nadal in the Italian Open quarter-finals, was able to avoid suffering the same fate as Thiem.
But he was made to work hard to do so against 27-year-old qualifier Otte, ranked 152 in the world.
Zverev immediately responded after Otte made the first break of serve in the contest to trail 4-3 - but he failed to hold serve once more as his compatriot went on to take a surprise first set.
Zverev's task was made even more difficult when a Otte won four consecutive games to close out the second set in impressive fashion.
But the momentum shifted as Zverev raced into a 5-1 lead on his way to comfortably claiming the third set - and consecutive breaks in the fourth set up a decider.
By then, a deflated Otte could offer no resistance as Zverev - who will face Russian Roman Safiullin or Spain's Carlos Taberner next - cruised to victory in the fifth.
The closing stages of the match were played without spectators because of France's curfew of 21:00 local time (20:00 BST) as part of coronavirus restrictions.
Tsitsipas overcomes slow start
Tsitsipas arrived at Roland Garros fresh from his Lyon Open win over Britain's Cameron Norrie a week earlier.
But there was no freshness about the start he made on Sunday against Chardy in an empty court Philippe Chatrier - again, because of local coronavirus restrictions.
First, he had to save a set point in the opening set. And he came back to win the tie break after having been 1-4 down.
From there, though, the Greek was much more businesslike as he comfortably took the following sets, wrapping up victory in a little over two hours.
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