Alexander Zverev, the ninth seed, was knocked out in the first round of the French Open by Fernando Verdasco.
The 20-year-old German was tipped to progress deep into the second week after tournament wins in Rome, where he beat Novak Djokovic in the final, and Munich.
However, in a match held over from Monday because of rain, Zverev had no answer to the left-handed Spaniard, going down 4-6 6-3 4-6 2-6.
Verdasco, 33, will now face Pierre-Hugues Herbert.
The veteran clay court specialist outmuscled the German when play resumed on Tuesday, with Zverev having lost his momentum from a day earlier when he had grabbed the second set and looked in complete control.
Meanwhile, home favourite and 12th seed Jo-Wilfried Tsonga has a battle on his hands against Argentine Renzo Olivo.
Olivo was serving for the match in the fourth set in the joint-latest ever finish at Roland Garros, but Tsonga broke back before play was suspended.
The duo will resume their epic encounter tomorrow, with the score at 7-5 6-4 6-7(8) 5-4 in Olivo’s favour, though it is on serve in the fourth after Tsonga’s late-night heroics.
Third seed Stan Wawrinka launched his bid for a second French Open titlle beating Slovak qualifier Jozef Kovalik 6-2 7-6(6) 6-3.
The 2015 champion struggled to time the ball on an overcast Court Suzanne Lenglen, frequently spraying his trademark backhand drive long or into the net as he looked to keep the points short against 152nd-ranked Kovalik, who was making his Roland Garros debut.
But the Swiss did enough to win the rallies that mattered, extricating himself from a deep hole in the second set tiebreak, taking the last five points after trailing 6-3.
After connecting well with a backhand to take the final point, a sigh of relief as he approached the net suggested he was glad to put the encounter – lasting a shade under two hours and in which he made 33 unforced errors – behind him.
Having had less time than he might have wished to adjust between Geneva, where he took the title on Saturday, and Paris, he took the longer view in upbeat comments after the match.
“It wasn’t particularly easy after Geneva..but I feel good,” Wawrinka said courtside. “I hope to go deep (into the tournament) this year.”
Argentine Juan Martin Del Potro beat compatriot Guido Pella 6-2 6-1 6-4 to book a second round spot. Del Potro served 13 aces to Pella’s three and dished out 33 winners as he sealed a comfortable victory.
The match was del Potro’s first at Roland Garros since 2012, when he lost in the quarter-final to Roger Federer.
The Swiss will play Ukraine’s Alexandr Dolgopolov in the second round.
Meanwhile, eighth seed Kei Nishikori recovered from a set down to beat Australian Thanasi Kokkinakis 4-6 6-1 6-4 6-4.
South Korea’s Chung Hyeon beat men’s 27th seed Sam Querrey 6-4 3-6 6-3 6-3 in the first round. Chung faces Uzbekistan’s Denis Istomin in round two.
Results from the French Open Men’s Singles Round 1 matches on Tuesday
Fernando Verdasco (Spain) beat 9-Alexander Zverev (Germany) 6-4 3-6 6-4 6-2
Chung Hyeon (South Korea) beat 27-Sam Querrey (U.S.) 6-4 3-6 6-3 6-3
Martin Klizan (Slovakia) beat Laurent Lokoli (France) 7-6(4) 6-3 4-6 0-6 6-4
3-Stan Wawrinka (Switzerland) beat Jozef Kovalik (Slovakia) 6-2 7-6(6) 6-3
Alexandr Dolgopolov (Ukraine) beat Carlos Berlocq (Argentina) 7-5 6-3 6-4
Denis Istomin (Uzbekistan) beat Ernesto Escobedo (U.S.) 7-6(3) 6-3 6-4
18-Nick Kyrgios (Australia) beat Philipp Kohlschreiber (Germany) 6-3 7-6(4) 6-3
29-Juan Martin del Potro (Argentina) beat Guido Pella (Argentina) 6-2 6-1 6-4
Kevin Anderson (South Africa) beat Malek Jaziri (Tunisia) 7-6(4) 6-3 7-6(4)
8-Kei Nishikori (Japan) beat Thanasi Kokkinakis (Australia) 4-6 6-1 6-4 6-4
Nicolas Almagro (Spain) beat Marcos Baghdatis (Cyprus) 6-7(3) 6-4 6-3 7-6(1)
Karen Khachanov (Russia) beat Nicolas Jarry (Chile) 6-4 3-6 7-6(4) 6-1
Jeremy Chardy (France) beat Radu Albot (Moldova) 6-2 6-4 7-6(3)
Paolo Lorenzi (Italy) beat Ricardas Berankis (Lithuania) 6-0 6-2 6-4
15-Gael Monfils (France) beat Dustin Brown (Germany) 6-4 7-5 6-0
Kyle Edmund (Great Britain) beat Gastao Elias (Portugal) 6-3 6-2 7-5
John Isner (U.S.) beat Jordan Thompson (Australia) 6-3 4-6 7-6(5) 6-3
13-Tomas Berdych (Czech Rep) beat Jan-Lennard Struff 6-1 6-1 4-6-4
Thiago Monteiro (Brazil) beat Alexandre Muller (France) 7-6(4) 2-6 4-6 7-6(3) 6-0