Wimbledon champion Serena Williams cemented her status as the favourite for Olympic singles gold with a crushing 6-1 6-0 victory over Russia's Vera Zvonareva to reach the quarter-finals.

The American, seeded fourth, needed only 51 minutes to cruise through under the Centre Court roof against the bronze medallist from Beijing four years ago.

Williams already has two gold medals to her name in women's doubles with sister Venus, and were she to match that in singles, she would join Steffi Graf in winning all four grand slam titles and the Olympics in her career.

In the last eight Williams will meet either eighth seed Caroline Wozniacki or Slovakia's Daniela Hantuchova.

Williams gave much of the credit to the Centre Court roof, saying: "This is like a dream court. Grass is a dream court, but then to play indoors on grass... oh my God it's crazy."

Kim Clijsters kept alive her hopes of a medal at her first and last Olympics by defeating 11th seed Ana Ivanovic of Serbia 6-3 6-4 on a packed Court 18.

The Belgian crowd favourite will retire for the second time after the US Open later this month but for the moment she seems to have put her injury troubles behind her.

Clijsters, who will now face either Maria Sharapova or Sabine Lisicki, feels she is coming into peak form just at the right time.

The Belgian said: "At this stage of the tournament, you have to fight and you have to play some of your best tennis, and I did. I felt like I was doing a lot of good things out there today, and I served a little bit better at the important points."

World number one Victoria Azarenka was given a real test by Russia's Nadia Petrova but she fought back from a break down in the second set to win 7-6 (8/6) 6-4.

Meanwhile, Maria Kirilenko's quarter-final opponent will be last year's Wimbledon champion, Czech Petra Kvitova, who was in superb form today as she crushed Italy's Flavia Pennetta 6-3 6-0.