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 2½ 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."
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."
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.
Williams took plenty of big swings against Kvitova, as though eager to hit winners and keep rallies short. She was 0 for 5 on break-point chances before she converted in the 10th game, taking the opening set when Kvitova netted a backhand.
A timid second serve gave Williams problems, but she repeatedly came up with clutch shots on break points, saving nine of 11. And in the final set, she pulled away.
"I think I'm a pretty good clutch player," Williams said, "so that third set is a good place for me to be."
Kvitova, who was slowed by a virus earlier this year, fell to 3-5 on North American hard courts since winning Wimbledon.
"It was good match, but with a bad end," Kvitova said.
No. 15 Ana Ivanovic lost only six points on her first serve and beat American Vania King 6-4, 7-5. Ivanovic was playing for the first time since she retired with a left hip injury a week ago at the Indian Wells semifinals.
No. 4 Andy Murray, the 2009 champion, beat Alejandro Falla 6-2, 6-3. Also advancing were No. 6 Jo-Wilfried Tsonga, No. 7 Tomas Berdych, No. 9 Janko Tipsarevic, No. 13 Gilles Simon, No. 16 Kei Nishikori and No. 18 Alexandr Dolgopolov.