SERENA WILLIAMS admits she will have to improve in the second week of Wimbledon if she is to lift her fifth title at SW19.
Williams wasted little time or energy in seeing off Barbora Zahlavova Strycova and Melinda Czink earlier this week, but the American was below par in her three-set thriller over China's Jie Zheng.
The 30-year-old lost the first set and needed to dig deep to see off the 25th seed and move into the last 16, where she will face Yaroslava Shvedova.
Little was known about Shvedova until she pulled off a golden set – winning 6-0 without dropping a point – against 10th seed Sara Errani on Saturday.
Much harder opponents await Williams in the latter stages of the tournament and the 13-time grand slam winner concedes she will have to improve to progress.
"I have to make sure I do better," Williams said. "Usually I return well but I hit so many errors off my returns against Zheng.
"I was just off against her, although she played unbelievable. I think I can play better."
Williams' main rival for the Championships, world No.1 Maria Sharapova, will move into the quarter-finals if she beats 15th seed Sabine Lisicki on Court No.1 today.
Petra Kvitova, meanwhile, has failed to repeat last year's Wimbledon success at other major tournaments and this week she began her title defence with a nervous win over Akgul Amanmuradova.
The Czech has improved since, losing just five games against Elena Baltacha and Varvara Lepchenko.
Like Williams, Kvitova reckons she must improve if she is to make Saturday's final, but she admits players are raising their games as she is defending champion.
"That's what I was like when I was ranked 60 in the world," said fourth seed Kvitova, who faces Francesca Schiavon today.
Second seed Victoria Azarenka takes on Ana Ivanovic on Centre Court while Agnieszka Radwanska faces Camila Giorgi on Court Three. Kim Clijsters, meanwhile, plays German Angelique Kerber.