KEY BISCAYNE, Fla. — The ball hung in the humid night air, so Venus Williams charged forward, using her long strides to arrive quickly and swat a winner.

KEY BISCAYNE, Fla. — The ball hung in the humid night air, so Venus Williams charged forward, using her long strides to arrive quickly and swat a winner.

Ever the aggressor, Williams took a big step in her comeback Friday night when she defeated reigning Wimbledon champion Petra Kvitova 6-4, 4-6, 6-0 in the second round of the Sony Ericsson Open.

Williams is playing in her first tournament since she withdrew from the U.S. Open last August after being diagnosed with a fatigue-causing autoimmune disease.

"I have definitely come a long way physically," said Williams, who is trying to pace herself by taking things easy when not on the court. "I definitely have to cut out a lot of extracurricular activities. I'm not going to be the one having fun dinners. I am going to be the one stuck in the room. The price to pay is definitely worth it."

She showed plenty of staying power against the No. 3-seeded Kvitova, taking charge in the final set. When Kvitova pushed a weary forehand wide on the final point after nearly 21/2; hours, Williams trotted to the net with a grin and raised fist.

"She's back," Kvitova said. "She looks very ready, very fit."

A former No. 1 and seven-time Grand Slam singles champion, Williams is ranked No. 134 and needed a wild card to enter the tournament, which she has won three times. She hopes to improve her ranking enough in the coming weeks to qualify for the London Olympics.

"I've got nothing to lose," Williams said. "Everything, every shot, is a victory and a blessing. So it's like I'm just, you know, going for it."

Defending champion Victoria Azarenka won her 24th match in a row and first since arriving at Key Biscayne when she beat Michaella Krajicek, 6-3, 7-5.

The top-ranked Azarenka's streak began with her first match this year, and her start is the best since Martina Hingis went 37-0 in early 1997.

"I'm not really thinking about numbers, the streaks, whatever," Azarenka said. "It's your job to count; mine is to play tennis."

Azarenka won her first Grand Slam title in January at the Australian Open, where she beat Maria Sharapova to earn the No. 1 ranking. She also has won titles this year at Sydney, Doha, and — just a week ago — Indian Wells.

When asked to give a reason for the streak, Azarenka wavered.

"There is not really an answer," she said. "There is one thing that I do, and it keeps happening for me: to win. There is really no magic."

On the men's side, No. 2 Rafael Nadal began a bid for his first Key Biscayne title by beating Santiago Giraldo 6-2, 6-0.

No. 10 seed John Isner overcame a slow start to beat Nikolay Davydenko 2-6, 6-3, 6-4. Davydenko retreated eight feet behind the baseline to better cope with the 6-foot-9 Isner's booming serves, but failed to break in the final two sets.

Alisa Kleybanova, playing in her first tournament since being diagnosed with Hodgkin's Lymphoma last May, lost to No. 22 Maria Kirilenko 7-6 (1), 6-3.

No. 15 Ana Ivanovic lost only six points on her first serve and beat American Vania King 6-4, 7-5.