WIMBLEDON, England >> Three times on a single point, Serena Williams hit a serve that clipped the net before plopping into the service box for a let, a sequence that drew giggles from the Centre Court crowd.

Otherwise there was no way to slow Williams on Wednesday. She hit 12 aces and repeatedly rocketed her groundstrokes past No. 13-seeded Vera Zvonareva to win 6-1, 6-0 and reach the Olympic quarterfinals.

Swinging lustily with almost every shot, the No. 4-seeded Williams hit 32 winners to three for the Russian, who also lost when they met in the 2010 Wimbledon final. Williams swept the final 10 games and was done in only 51 minutes.

“I was just playing unbelievable,” Williams said. “I was nervous going into the match and I didn’t speak to anyone and I had a bad practice. I had no idea I would play like this.”

Roger Federer endured two rain delays and a shaky moment late in the first set to reach the quarterfinals by beating Denis Istomin of Uzbekistan 7-5, 6-3.

Serving at 5-all in the first set, the No. 1-seeded Federer faced three break points and erased them all. He then broke and was in control from there.

A four-time Olympian, Federer has yet to win a singles medal, although he and Swiss teammate Stanislas Wawrinka did win the gold in doubles in 2008. Federer is playing his first tournament since winning a record-tying seventh Wimbledon title.

Also Wednesday, No. 2 Novak Djokovic hit 16 aces and came from behind to beat Lleyton Hewitt of Australia 4-6, 7-5, 6-1. Djokovic next plays No. 5 Jo-Wilfried Tsonga of France, who beat Feliciano Lopez of Spain 7-6 (5), 6-4 less than 24 hours after winning a 48-game third set.

Four-time Grand Slam champion Kim Clijsters, playing in her first Olympics only weeks before she retires, beat former No. 1 Ana Ivanovic of Serbia 6-3, 6-4. Top-seeded Victoria Azarenka of Belarus edged No. 16 Nadia Petrova of Russia, 7-6 (6), 6-4.

While play was interrupted by rain on other courts, Williams worked under Wimbledon’s roof, and the conditions clearly suited her. She whacked winners even from behind the baseline, and more than once spun on one foot after smacking a shot, mixing a little body English with plenty of brute force.

Williams hit a Wimbledon record 102 aces en route to her fifth title at the All England Club last month, and the shot remains a dominant force, especially on grass. She hit three consecutive aces against Zvonareva and lost only seven points in six service games.

In the second set, the crowd tried to coax a comeback from the Russian. But when she began tossing her racket in frustration, cheers for her turned to jeers.

Williams has lost only 10 games through three rounds. The three-time Olympian is the winner of two gold medals in doubles and 14 major titles, but she has yet to win a singles medal.

She’s on course for a semifinal showdown against her sister Venus, who won a second-round match Tuesday.

“I was inspired by her,” Serena said. “I watched her and I said, ‘Serena, you need to play better, or she’s getting the gold.'”