Paris - Simona Halep became the highest seeded player to exit the French Open so far as last year's runner-up lost 7-5, 6-1 to Mirjana Lucic-Baroni in the second round on Wednesday.
The third-seeded Halep, who reached her first major final at Roland Garros in 2014, struggled from the start on Court Suzanne Lenglen and was overpowered in the second set, slumping to her second straight defeat against her 70th-ranked opponent.
The Croatian had also beaten Halep at the US Open last year.
"It was difficult because I didn't feel the game," Halep said.
"I don't have a good feeling of my game, and that's why I couldn't change something."
Lucic-Baroni hit an ace on match point and raised both arms in delight.
She finished with 29 winners, compared to five for Halep.
"It's incredible, she's just a great champion and I respect her so much," Lucic-Baroni said.
"It's such a huge win."
Up next for the Croatian is local favorite Alize Cornet.
"I wish at least three people will cheer for me in that match," Lucic-Baroni said.
Earlier, defending champion Maria Sharapova limited her mistakes to just eight unforced errors as she beat Russian Fed Cup teammate Vitalia Diatchenko 6-3, 6-1.
She will now get an early test in her bid for a third French Open title, having set up a third-round match against 2010 French Open finalist Samantha Stosur.
The Australian swept to a 6-0, 6-1 victory over French wild card entry Amandine Hesse.
The rout extended Stosur's winning streak to seven matches after she arrived in Paris on the back of her first title this year in Strasbourg.
In men's play, second-seeded Roger Federer made light work of Marcel Granollers, winning 6-2, 7-6 (1), 6-3, while 2014 Australian Open winner Stan Wawrinka reached the third round with a 6-3, 6-4, 5-7, 6-3 defeat of Dusan Lajovic.
No. 5 Kei Nishikori, No. 12 Gilles Simon and 14th-seeded Jo-Wilfried Tsonga advanced in straight sets.
Gael Monfils was given a hard time on centre court by Diego Schwartzman before rallying past the 62nd-ranked Argentine 4-6, 6-4, 4-6, 6-2, 6-3.
The 26th-seeded Stosur has not beaten Sharapova since a match on hard court in Tokyo three years ago, with Sharapova boasting a 14-2 career record against the Australian veteran.
"Playing Maria is always a big challenge for me," Stosur said.
"No matter what surface it's on, I don't have a very good record at all."
But Stosur can take confidence from their previous match on the Parisian clay in the fourth round last year, when she led 6-3, 4-3 before Sharapova won the next nine games.
"It's one of those matches that's a tough matchup, but I know I've got the game that can trouble her, and hopefully I can do it well and we will see what happens," Stosur said.
After reuniting last month with former coach David Taylor, Stosur is hitting form at the right time. Before Strasbourg, where the 2011 US Open champion won a seventh career title, Stosur had won back-to-back matches just once in 10 tournaments.
She said her good spell of form is mainly due to her renewed partnership with Taylor.
"I think going back with Dave, that's given me confidence," Stosur said.
"That's probably a contribution, and then playing on a surface that I feel good on. Been able to get over a couple of injuries again.
"It all I guess makes for a better kind of couple of weeks."
Sharapova's main focus at the moment is to fully recover from her cold after winning the Italian Open.
"For me right now it's really about recovery and just being healthy for the next round," said Sharapova, who is bidding to become the first player to retain her title in Paris since Justine Henin in 2007.
She also won in 2012.
"Fortunately I played a lot of tennis in the previous two tournaments and this is not a stage where you're trying to fix things or work on things. It's really about maintenance and recovery and getting ready for the next match," she said.
Among other seeded women to advance were No. 11 Angelique Kerber, No. 13 Lucie Safarova and No. 20 Sabine Lisicki.
Results from the fourth day of the French Open at Roland Garros on Wednesday (x denotes seeded player):
Gilles Muller (LUX) bt Paolo Lorenzi (ITA) 4-6, 4-6, 7-6 (7/1), 7-6 (7/5), 6-4
Gilles Simon (FRA x12) bt Martin Klizan (SVK) 7-5, 6-2, 6-3
Steve Johnson (USA) bt Sergiy Stakhovsky (UKR) 2-6, 6-3, 7-6 (7/5), 7-6 (8/6)
Stanislas Wawrinka (SUI x8) bt Dusan Lajovic (SRB) 6-3, 6-4, 5-7, 6-3
Roger Federer (SUI x2) bt Marcel Granollers (ESP) 6-2, 7-6 (7/1), 6-3
Benjamin Becker (GER) bt Fernando Verdasco (ESP x32) 6-4, 0-6, 1-6, 7-5, 10-8
Teimuraz Gabashvili (RUS) bt Juan Monaco (ARG) 6-3, 6-4, 6-2
Damir Dzumhur (BIH) bt Marcos Baghdatis (CYP) 6-4, 6-3, 4-6, 6-2
Kei Nishikori (JPN x5) bt Thomaz Bellucci (BRA) 7-5, 6-4, 6-4
Pablo Cuevas (URU x21) bt Dominic Thiem (AUT) 7-6 (9/7), 7-5, 6-7 (5/7), 7-5
Lukas Rosol (CZE) bt Roberto Bautista Agut (ESP x19) 6-4, 6-2, 6-2
Benot Paire (FRA) bt Fabio Fognini (ITA x28) 6-1, 6-3, 7-5
Nicolas Mahut (FRA) bt Ernests Gulbis (LAT x24) 6-3, 3-6, 7-5, 6-3
Jo-Wilfried Tsonga (FRA x14) bt Dudi Sela (ISR) 6-4, 6-1, 6-1
Gael Monfils (FRA x13) bt Diego Schwartzman (ARG) 4-6, 6-4, 4-6, 6-2, 6-3
Tomas Berdych (CZE x4) bt Radek Stepanek (CZE) 6-3, 6-7 (7/9), 6-3, 6-3
Annika Beck (GER) bt Paula Kania (POL) 6-2, 6-2
Lucie Safarova (CZE x13) bt Kurumi Nara (JPN) 6-2, 6-0
Sabine Lisicki (GER x20) bt Daria Gavrilova (AUS) 6-1 - retired
Samantha Stosur (AUS x26) bt Amandine Hesse (FRA) 6-0, 6-1
Maria Sharapova (RUS x2) bt Vitalia Diatchenko (RUS) 6-3, 6-1
Alize Cornet (FRA x29) bt Alexandra Dulgheru (ROM) 6-2, 7-5
Mirjana Lucic-Baroni (CRO) bt Simona Halep (ROM x3) 7-5, 6-1
Donna Vekic (CRO) bt Bojana Jovanovski (SRB) 6-4, 6-3
Angelique Kerber (GER) bt Ajla Tomljanovic (AUS) 6-3, 6-2
Elena Vesnina (RUS) bt Polona Hercog (SLO) 2-6, 6-1, 6-3
Elina Svitolina (UKR x19) bt Yulia Putintseva (KAZ) 1-6, 7-5, 6-4
Garbie Muguruza (ESP x21) bt Camila Giorgi (ITA) 6-1, 6-4