Serena Williams tops Andrea Petkovic in Stanford semis

Stanford: Serena Williams has put to rest any lingering concerns about her illness-caused departure from Wimbledon with a strong showing in her hard-court opener.

Williams reached her fourth final of the season by taking the last eight games to beat Andrea Petkovic 7-5, 6-0 on Saturday in the Bank of the West Classic.

The top-seeded and top-ranked Williams has won all three finals she has contested so far in 2014, winning titles at Brisbane, Miami and Rome.

The tournament is Williams' first since she tried to play through a viral infection that caused her to lose equilibrium during a doubles match with sister Venus at Wimbledon.

In Sunday's final at the hard-court tournament, Williams will face third-seeded Angelique Kerber, a 4-6, 7-6 (4), 6-2 winner over Varvara Lepchenko.

Williams turned in her best performance yet against the eighth-seeded Petkovic, striking the ball with confidence and surviving long rallies.

"I'm feeling healthy, I'm feeling good and more than anything I'm feeling positive," Williams said. "I started making shots, got into a rhythm and it all seemed to come together for me."

Williams had six aces and one double fault while playing consistently well. She held serve throughout and only needed to save one break point.

"I still think I can do some things better," said Williams, who will reach her 200th non-consecutive week at No. 1. "It's important to win these matches to try and get on the right track."

Williams has already defied the odds as the oldest player to be ranked No. 1 in the world. She also turned in the best season of her career last year, winning a career-best 11 titles.

Since returning from a right foot injury and pulmonary embolism in 2011, winning her first title in more than a year at Stanford, she's won 23 of her 60 titles. She needed 12 years to win her first 37.

"I was thinking recently about appreciating what I've done. I just don't do it enough," she said. "I've done everything I've needed to do, so why not enjoy it? Why not have fun? I don't do that enough."

Against the ninth-ranked Petkovic, Williams wasted four break points in the final game of the first set before eventually converting her fifth.

"I had to step up my game," Williams said. "Andrea was playing so well and making her shots. I just thought I needed to play better or be down a set."

Petkovic was trying to become the ninth player - and first since 2010 - to beat both Venus and Serena Williams in one tournament. Petkovic eliminated Venus in the quarterfinals Friday.

"Playing one of them is a lot," Petkovic said. "Playing two of them in a row was so strange, like playing a video game. I would have beaten a lot of other players the way I played in the first set."

Petkovic also said she started the second set on a good note.

"I made four decent serves and she fired them right back at me," Petkovic said. "I started hurrying after that."

A spectator fell out of the stands during the first set of the Kerber-Lepchenko match, delaying it a few minutes. The spectator was able to walk, with some assistance, out of the stadium.