Victoria Azarenka was knocked out of the French Open fourth round on Sunday and could now lose her world number one ranking, while Novak Djokovic narrowly avoided following her to the exit.
Belarussian Azarenka was beaten 6-2 7-6 by Slovakian 15th seed Dominika Cibulkova and must wait to see if Russian Maria Sharapova will claim the top spot.
Sharapova, who meets unseeded Czech Klara Zakopalova in the last 16 on Monday and has yet to drop a set here, must get to the Roland Garros final for the first time to become number one.
Cibulkova, who reached the French semi-finals in 2009 and had lost seven times to Azarenka in eight previous meetings, collapsed on to her back, wearing a big grin, after winning the tiebreak 7-4 with a backhand crosscourt on her second matchpoint.
The Slovakian will play US Open champion Samantha Stosur of Australia who joined the quarter-final line-up late on Sunday by beating American teenager Sloane Stephens 7-5 6-4.
A stone-faced Azarenka, asked by a reporter how she would recover from her defeat, said sarcastically: "I'm going to kill myself!"
Azarenka, who had come within five points of defeat in the first round against Italy's Alberta Brianti, was also asked what had gone wrong on Sunday.
"Pretty much everything, really," she said. "I don't know how to describe my performance today...I wasn't satisfied being out there playing that way but I guess it happens."
Djokovic, the men's world number one, had to come back from two sets down to beat Italian Andreas Seppi and continue his quest to hold all four grand-slam titles at once.
The Serbian hit 77 unforced errors - 26 more than defending champion Rafa Nadal had racked up in three matches - before turning things around and beating Seppi 4-6 6-7 6-3 7-5 6-3.
"It was one of those days where...nothing is working really," Djokovic told a news conference. "My serve was fine but aside from that I could not get into the rhythm. I was fighting, and I think because of the fight I won the match.
"All the congratulations for his tournament and performance today. I think he was a better player for the first two sets, definitely.
"I was very fortunate to come through this match," added Djokovic who will now play either fifth-seeded Frenchman Jo-Wilfried Tsonga or number 18 Stanislas Wawrinka of Switzerland.
Roger Federer dropped the first set against Belgian fan and newcomer David Goffin before imposing his authority to reach the quarter-finals 5-7 7-5 6-2 6-4.
The Swiss, champion in 2009, saved two breakpoints in the first set before hitting a forehand error on the third but gradually began to dominate the match.
The cool, windy weather had made conditions difficult on Suzanne Lenglen Court, Federer said.
"I really struggled for a long time out there and conditions were slow," he told a news conference.
Lucky loser Goffin, the first man in 12 years to reach the last 16 on his Grand-Slam debut, impressed Federer and the crowd with his fighting spirit and prowess at the net, bowing to the fans on three sides after one particularly athletic dropshot.
Federer will now play either seventh seed Tomas Berdych or number nine Juan Martin del Potro and said he would be glad to meet a more familiar player.
"It has been a tough last couple of weeks, I have played guys I have almost never heard about and I don't know their patterns and it's made things difficult for me," he said. "But I have made it through and I will definitely meet someone I know a lot better."
Italy's Sara Errani saw off her second French Open champion in two rounds when she beat Russian Svetlana Kuznetsova 6-0 7-5 to reach the quarter-finals of the claycourt grand slam for the first time.
"I am curious to see how far I can go, what level I can get to," said clay specialist Errani, who had knocked out Serbian Ana Ivanovic, the 2008 winner, in the third round.
Kuznetsova's demise left Li Na as the only French Open champion still in the women's draw, after earlier-round defeats for Francesca Schiavone and Serena Williams.
China's Li, who won last year, meets Kazakh qualifier Yaroslava Shvedova in the fourth round on Monday.
Kuznetsova, who won in 2009, said Sunday's gloomy weather had affected her.
"Today the weather was so bad. I felt cold during the whole match. For me to move was really complicated, I couldn't make my feet move," said the Russian who started working with Marat Safin's old coach Hernan Gumy a week before the tournament.
Errani will now play another first-time quarter-finalist here, 10th seed Angelique Kerber who beat Croatian Petra Martic 6-3 7-5.
The Italian said she would change her tactics to play left-hander Kerber but, pressed on what she would do, told reporters with a smile: "I am not telling you for now, I am keeping it to myself."
Stosur had to play on Court One, after men's matches on the main showcourts ran longer than expected, and took her time to finish off her win.
She was broken as she served for the match at 5-3, perhaps disturbed by having to replay a point when a pigeon swooped along the net, but broke Stephens to love in the next game.