Holder Petra Kvitova suffered a shock 3-6, 7-5, 6-4 defeat by Serbia's former World No 1 Jelena Jankovic who produced an inspired display in the Wimbledon third round on Saturday.
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Second seed Kvitova breezed through her first two matches but after taking the first set against 30-year-old Jankovic, the Czech made too many errors allowing the 28th seed to triumph.
"I am overwhelmed and so excited," said Jankovic, who has never been past the fourth round at the All England Club and admits that grass is not her favourite surface.
"I have played so poorly the last couple of years and playing on Centre Court against the defending champion was unbelievable. The crowd was really great.
"My opponent was in great form, no doubt about it, I just tried to stay in there, stay positive and fight and I made it. I have to keep going."
Twice champion Kvitova looked confident at the start, having dropped only three games in her first two matches, and when she broke Jankovic in the fourth game she looked set to progress.
The 25-year-old broke again in the third game of the second set on a sun-drenched Centre Court and went 4-2 ahead as Jankovic struggled to match her power around the court.
But the experienced Serb had battled through two three-set matches already at the tournament and knew how to dig deep as she fought back to 4-4 before forcing a decider.
The final set went with serve until Jankovic broke Kvitova to close out the match.
"I am glad I was able to win against a two-time Wimbledon champion," added Jankovic.
"I could not believe it. Beating Petra gives me a lot of confidence for the tournament."
In the last 16 Jankovic faces Poland's 13th seed Agnieszka Radwanska who beat Casey Dellacqua of Australia 6-1, 6-4.
Kvitova, who won the Wimbledon title in 2011 as well as last year, was at a loss to explain how she had gone from a set and 4-2 up to losing a match that looked well within her control.
"Tough to explain. If I knew what happened, I would tell you. But I'm not really sure what happened out there. I was kind of up in the second set. Suddenly I felt like she's, you know, coming back, playing a little bit aggressive," said Kvitova.
"Suddenly from my side, I didn't have an answer for it. My serve didn't help me at all this time, as well. I was really struggling with each shot which I played."