Li Na of China hits a return against Barbora Zahlavova Strycova of the Czech Republic during their women's singles tennis match on Court 1 at the Wimbledon Tennis Championships in London, June 27, 2014. [Photo/Agencies]

LONDON - World number two Li Na became the biggest seed casualty at the Wimbledon as she was shocked by Barbora Zahlavova Strycova of the Czech Republic 7-6 (7-5), 7-6 (7- 5) at the third round on Friday.

The match was pretty much close but it was Zahlavova Strycova who held her nerve when it mattered most, hitting powerful shots around the baseline to frustrate and eventually overcome Li in two tense tie-breaks.

"I think I made a wrong decision. I need to play some matches before the Wimbledon. I think sometimes I don't know how to play the point, especially in the important moment. I think I made a lot of errors," said Li.

"I always wait, wait to see if the opponent makes mistakes, but today it didn't work."

The victory handed Zahlavova Strycova a place in the fourth round of a major for the first time in her career. There she will face No 16 seed Caroline Wozniacki.

Former world number one Wozniacki reached round four with a 6-3, 6-0 defeat of Croatia's Ana Konjuh.

Elsewhere, Petra Kvitova won an epic battle against five-time champion Venus Williams 5-7, 7-6 (7-2), 7-5.

After the match, Williams looked tearful as she waved farewell to the crowd, but she denied any talk of retirement.

"There have been people trying to retire me since I was 25," she said. "For some reason, we do that in tennis to our players. It's weird. We don't encourage them to stick around. It's like, 'Get out of here'."

"So I'm not getting out of here. I think this year has been a great year. I've had some tough losses, but I've learned a lot from them. I'm finding my way back on my feet.

"I'm proud of myself for what I'm achieving on the court."

Fourth seed and 2012 runner-up Agnieszka Radwanska trounced Michelle Larcher de Brito 6-2, 6-0 and the Pole will play Russia's 22nd seed Ekaterina Makarova, who beat Caroline Garcia 7-5, 6-3.

Romania's third seed Simona Halep, runner-up at this year's French Open, reached third round with a 6-3, 4-6, 6-4 victory over Ukraine's Lesia Tsurenko.

Halep, 22, will play world number 71 Belinda Bencic next after the Swiss player beat American Victoria Duval 6-4, 7-5.