It is one of the those infectious smiles Jelena Jankovic unashamedly admits she cannot wipe from her face hours after dethroning defending champion Petra Kvitova on Centre Court.

A former No.1 and a semi-finalist or better at each of the other three Grand Slams, a third-round victory on the grass would not normally elicit such a euphoric response.

But it is seven years since the Serbian reached her only Grand Slam final, in New York, and the 30-year-old has been teetering on the brink of tumbling outside the top 30.

Against the world No.2, a player who had dropped just three games total in her previous two rounds on a surface that brings out her attacking best, Jankovic was a heavy underdog.

She has never before managed to crack the fourth round at the All England Club and had this week battled through drawn-out three setters just to make it this far.

By Saturday afternoon she had left a two-time champion’s title defence in tatters, dropping to her back when Kvitova dumped one final limp backhand halfway up the net.

“I cannot stop smiling. I'm really, really happy,” she beamed. “I was just happy to be playing and competing on grass. Grass is not my favourite surface and Petra plays so well on it. I think it's her favourite surface. It shows by winning two Wimbledon titles.”

The Czech No.2 seed was in imperious form and looked to be continuing in the same vein when she raced through the first set 6-3 and up a break 3-1 in the second. But from there the free-swinging lefty’s game began to desert her and the Jankovic hung tight to land her first break, levelling at 4-4.

She went on to break Kvitova’s serve on her third set point when the Czech’s forehand found the net.

Coming into their seventh career meeting Jankovic had won two of their meetings but not since Doha a year ago.

Having played her way into a winning position, Kvitova found it difficult to explain just how the tables could turn so abruptly.

“I'm not really sure what happened out there. I was kind of up in the second set. Suddenly I felt like she's coming back, playing a little bit aggressive,” Kvitova said. “Suddenly from my side, I didn't have answer for it. My serve didn't help me at all this time, as well. I was really struggling with each shot.”

Pressing to land the break of serve at 4-4 in the decider, Kvitova stopped play to challenge a baseline call, despite appearing to have won the point with her next backhand crosscourt. It proved costly. The ball was shown to have caught the baseline and Jankovic went on to hold for 5-4.

Clearly rattled by her error of judgment, Kvitova faded. She sent a forehand metres long to give her 30-year-old opponent match point before dumping a limp backhand into the net to end her campaign before the quarter-finals for the first time since 2009.

“I took a challenge because I really thought it was out,” Kvitova explained. “That surprised me unfortunately in a bad way. It was still 30‑all and was a good chance to, you know, have a break. But unfortunately she served well and I didn't make it. Not to be in the second week of the favourite tournament for me is really sad. I don't really know what I can say.”

For the No.28 seed Jankovic, it will be the fifth time she has reached the fourth round and sets up a meeting with former finalist, No.12 seed Agnieszka Radwanska.