Serena on song at Stanford

Stanford - Serena Williams put a worrying Wimbledon behind her on Wednesday, launching her build-up to the US Open with a 7-5, 6-2 victory over Karolina Pliskova at Stanford.

The American world number one, playing for the first time since her third-round singles exit at Wimbledon was followed by a dramatic doubles retirement, showed no sign of illness or rust as she rolled to victory over the 45th-ranked Czech in 64 minutes.

After enjoying a first-round bye, the top seed punched her ticket to the quarter-finals of the WTA hardcourt tournament, where she'll face the winner of Thursday's second-round match between former world number one Ana Ivanovic and Canadian qualifier Carol Zhao.

Second-seeded Agnieszka Radwanska was less successful in her first outing since a disappointing Wimbledon.

Unseeded Varvara Lepchenko toppled the world number five 6-3, 3-6, 6-4 for her first win over the Polish star in six career meetings.

Radwanska, in her first match since a fourth-round Wimbledon exit, squandered a 3-1 lead in the opening set.

She broke Lepchenko three times to take the second set, but was on the defensive after an early break in the third.

Radwanska broke back to knot the set at 3-3, but Lepchenko, ranked 59th in the world, broke again for a 5-4 lead and seized her chance for a first career victory over a top-five player, holding serve in the final game to seal the triumph after two hours and 35 minutes.

With the US Open due to start on August 25, it was another disappointing outing for Radwanska.

She has yet to lift a trophy in 2014, and hasn't reached a final since a runner-up finish to Italian Flavia Pennetta at Indian Wells in March. Lepchenko next faces US qualifier Sachia Vickery, a 6-7 (4/7), 6-2, 6-1 winner over Puerto Rico's Monica Puig.

After falling in the third round of singles play at Wimbledon, Williams looked wobbly in a second-round doubles match from which she and her sister Venus eventually retired.

Since then Serena had withdrawn from a tournament in Bastad, but she said this week her time off had helped her recover from a virus and she showed no ill-effects on Wednesday.

"She had a really good serve," Williams said of Pliskova, who kept the first set close but finally surrendered the frame when Williams broke her in the final game.

Williams broke again in the second game en route to a 3-0 lead in the second, and broke Pliskova to love to close out the match when the Czech belted a forehand long.

Williams saved all three break points she faced in the match and fired nine aces.

"I think I played really well," Williams said. "I'm on the right path. I'm excited and I'm looking forward to my next match."