Fifth seed Caroline Wozniacki was dumped out of the French Open second round as she lost 6-4 7-6 (7/4) to Germany's Julia Goerges.
Wozniacki has now failed to reach round three in Roland Garros since 2012 as the Dane was out-powered by a superb Goerges performance.
Goerges, ranked 72nd in the world, has won all three meetings between the pair on clay and Wozniacki admits she has a mental block when she faces the German.
"Especially on clay she has a way to make me feel like I'm not playing very well," Wozniacki said.
"There are just some players that it's a tough match-up for you no matter the rankings, and some of them are easy match-ups no matter the rankings.
"She has given me trouble in the past and she gave me trouble again today, so that kind of sucks."
Goerges, who will now play American Irina Falconi, has never gone further than the fourth round at a Grand Slam but she believes her style of play is difficult to combat on clay.
"I don't play the typical women's game on clay court," Goerges said. "I play a little bit more spin, a little bit higher over the net than some of the other girls.
"When she gets the ball in her striking zone she doesn't miss any balls, but if you give her different balls then it's tougher for her.
"It's never a guarantee you're going to win the match, you have to execute as well, and that's what I did mainly today very well."
Also through to the third round is Italian Francesca Schiavone, who emerged victorious after another marathon contest with Russian 18th seed Svetlana Kuznetsova.
The pair played the longest ever women's Grand Slam match in the last 16 of the Australian Open four years ago, which Schiavone also won, and she had to dig deep again to claim a 6-7 (11/13) 7-5 10-8 victory.
The match lasted three hours and 50 minutes, short of their record four hours 44 minutes in 2011, while the final set lasted 94 minutes and included nine consecutive breaks of serve.
"She played unbelievable points," Kuznetsova said.
Shortly after Wozniacki was knocked out by Goerges - meaning the third, fifth and sixth seeds had all gone before round three - Serena Williams (33) faced unheralded 21-year-old German Anna-Lena Friedsam.
It looked like a mismatch but 105th-ranked Friedsam gave the 19-times Grand Slam champion a torrid time, taking the first set before a nervy Williams recovered to scrape into the third round 5-7 6-3 6-3.
"A win is a win and as long as you live to survive the next day, you can always improve," Williams said. "I know my level is literally a hundred times better than I played today.
"I take more solace in the fact I can play better as opposed to the fact that that's the best I could play - then I would be in trouble."
She will have to sharpen up against former world No 1 Victoria Azarenka of Belarus in the next round.
In the men's event, Rafael Nadal and Novak Djokovic remain on course for a quarter-final clash after both players recorded straight-sets victories in the second round.
Nadal eased past fellow Spaniard Nicolas Almagro 6-4 6-3 6-1 while Djokovic overcame a problem with his hip to get the better of Luxembourg's Gilles Muller 6-1 6-4 6-4.
It means Nadal, who is bidding to clinch his 10th title at Roland Garros, is now only two wins away from the last eight, where he could meet Djokovic, who is aiming to lift his first title in Paris.
The Serb had to call for treatment during his win but he insists the problem is not serious.
"It's not a concern for the next match, which is the most important thing obviously," Djokovic said.
"A little bit heavier conditions made the court a little bit more wet and it was pretty slippery. So I think that's when it happened, I made a couple of slides that were quite unusual, with the change of directions.
"And those dynamic movements, jammed the hip a little bit. Thankfully it's nothing serious. It's going to be fine."
Djokovic will now play Australia's Thanasi Kokkinakis, who came from two sets down to beat compatriot Bernard Tomic 3-6 3-6 6-3 6-4 8-6.
Andy Murray is also through after beating Portugal's Joao Sousa 6-2 4-6 6-4 6-1 and the Scot will now play Krygios in round three.
There were also wins David Ferrer, Marin Cilic, David Goffin and Jack Sock but 16th seed John Isner is out after the American lost to France's Jeremy Chardy.
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