Top seed Alexander Zverev crashed out in his opening match of the Monte Carlo Masters on Tuesday, falling to Italy's Matteo Berrettini as defending champion Stefanos Tsitsipas and Britain's Jack Draper both advanced.
World number two Zverev has struggled for any kind of form since his defeat by Jannik Sinner in the Australian Open final at the end of January, winning just six of 12 matches.
But with a first-round bye in the absence of world number one Sinner, who is suspended for a doping violation until May 4, Zverev was among the favourites.
After a controlled start to the game, the German slumped to a 2-6, 6-3, 7-5 defeat to 34th-ranked Berrettini, at the end of a high-flying match, which had included an impressive 48-stroke rally won by the Italian.
"It's been the worst period since my injury (the) last few months," said Zverev who has suffered a string of early exits of late, including at Indian Wells.
"I played a great first set, and once I got broken in the second set I play ten levels down. My ball is much slower. I stop hitting the ball.
"The same story the last few months. Nothing changes. So it's me who lost the match, once again.
"I thought my level was terrible, but that's just my opinion."
Defending champion and three-time Monte Carlo winner Tsitsipas advanced past 38th-ranked Australian Jordan Thompson. After a lacklustre US tour, the Dubai winner won through 4-6, 6-4, 6-2 in his second round tie.
Indian Wells champion Draper, seeded fifth, eased past 45th-ranked American Marcos Giron 6-1, 6-1.
Novak Djokovic -- the winner in 2013 and 2015 -- and Carlos Alcaraz start their campaigns on Wednesday.
'COMFORTABLE ON CLAY'
Zverev won the first set against former Wimbledon runner-up Berrettini but a break of serve in the sixth game of the second set allowed the Italian to level the match.
Berrettini broke at 3-all in the final set before blowing a chance to close out victory on his own serve, but he broke again for a 6-5 lead after an astonishing 48-shot rally.
The 34th-ranked Berrettini made no mistake with his second opportunity though, advancing to a last-16 meeting with Lorenzo Musetti or Jiri Lehecka.
"The game plan was the same but I changed my attitude and the way I was believing in my strokes," Berrettini said.
"I told myself to be more aggressive and if I am going to lose this match, I am going to do the right things and luckily it worked."
Berrettini has won 17 of his past 18 matches on clay. Last season, he captured clay-court titles in Marrakesh, Gstaad and Kitzbuehel. However, he hasn't played at the French Open since 2021 due to injuries.
"I have missed the biggest tournament on clay for the past three years and that was tough and now I want to enjoy it. I feel really comfortable on clay," said the former world number six.
For Zverev, last year's Roland Garros runner-up, it was another disappointing outcome on clay after quarter-final exits in Buenos Aires and Rio de Janeiro. His next tournament will be on home soil in Munich.
Meanwhile Tsitsipas let a 5-2 lead slip in the second set, before getting past Thompson to set up a third round meeting with either Portugal's Nuno Borges or Spaniard Pedro Martinez.
"I really didn’t know what to expect, you don’t know what your opponent is capable of," said Tsitsipas.
"He showed a good first set, he seemed to be playing quite reserved and wasn’t giving me much to work with. I was just trying to find something to reignite that consistency within my game.”
Dane Holger Rune, the 10th seed, retired due to illness against Portugal's Nuno Borges.
Rune, the Monte Carlo runner-up in 2023, called for the doctor after losing the opening set before shaking hands with his opponent while trailing 6-2, 3-0.
Bulgarian Grigor Dimitrov, a two-time Monte Carlo semifinalist, defeated Chilean Nicolas Jarry 6-3, 6-4, on his seventh match point.