KEY BISCAYNE, Fla. — The ball hung in 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.

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 2Z\x hours, Williams trotted to the net with a grin and raised fist.

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.

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.