Dominika Cibulkova of Slovakia downed top seed Victoria Azarenka of Belarus to pull off a major upset in fourth round action at the French Open on Sunday.
The 15th seed won 6-2, 7-6 (7/4) to set up a quarter-final match with either Samantha Stosur of Australia or Sloane Stephens of the United States.
Azarenka won the Australian Open in January and had been expected to coast past Cibulkova having beaten her seven times out of eight previously.
But the little Slovakian, who reached the semi-finals in 2009, handled the cold, windy conditions on the Suzanne Lenglen court far better than the tall Belarusian.
After losing the first set, Azarenka appeared to be getting back on track as she broke to lead 2-0 in the second, but 23-year-old Cibulkova dug deep to run off four games in a row.
Azarenka levelled at 4-4, with the set then going to a tie-break which a pumped-up Cibulkova led from the start to pull off a stunning win.
"I am getting more mature and more tough mentally," said Cibulkova, explaining that she had learned from the experience of losing to Azarenka in Miami earlier this year when leading a set and 5-2.
"I managed to go through these emotions. She was 6-5 up, and I said, hey, come on, you have to play your game again and just make it.
"And for the tiebreak I'm very, very proud of myself that I was still going for my shots, and I just made it because she would never give me the match.
Azarenka said: "It wasn't satisfying at all, not satisfying being out there playing like that. I do not know what to find positive in that. No excuses, just a bad performance."
Earlier Sara Errani of Italy and Angelique Kerber of Germany both reached the quarter-finals at Roland Garros for the first time with straight sets wins.
Errani, seeded 21, defeated Svetlana Kuznetsova of Russia 6-0, 7-5, while 10th seed Kerber ousted Petra Martic of Croatia 6-3, 7-5.
The pair will meet in the last eight in a top half of the draw which had also contained Azarenka.
Errani has been the form player on clay this year winning tournaments in Acapulco, Barcelona and Budapest. She also reached the Australian Open quarter-finals in January on hardcourts.
After a tough opening game, Errani took total control as 2009 champion Kuznetsova struggled to put her game together.
The first set was completed in just 30 minutes and it was more of the same at the start of the second as Errani, one of the smaller players on the circuit at just 1.64m, used her mobility and claycourt expertise to good effect.
Errani moved 2-0 up before Kuznetsova finally found some form, breaking the Italian to love to open her account.
The Russian levelled at 2-2 and had a break point in the next game to move ahead, but Errani saved that with a blistering forehand winner at the end of a long rally.
Two more break points followed two games later and Kuznetsova converted the first of these to move 4-3 up. She then served for the set at 5-4 up but was broken to love.
Errani took the next two games to reach her first French Open quarter-finals.
Kerber, a semi-finalist at the US Open last year, easily won the first set against Martic and she immediately got a break in the second when the Croatian, ranked 50th in the world, miscalculated a drop shot and hit the ball into the net.
Martic broke back in the eighth game to level the set and held her serve to take the lead but in the following game, Kerber fired off an ace to level the score at 5-5.
In the next game, Kerber prevailed in a 28-shot rally to go to break point and then sliced in a deft drop shot to move ahead 6-5.
The left-handed Kerber finally secured her quarter-final place on her third match point with a forehand winner on a poor return by the Croatian, who suffered nine double faults on her way to elimination.
Asked what was the reason behind her sudden charge up the world rankings in the last 12 months Kerber replied: "After the US Open I started to believe in myself more and I started to do more practice and fitness things.
"I know that I can beat the top players and now I am one of them. I think that nobody wants to play against me right now."