Venus Williams took a big step in her comeback 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 US Open in August after being diagnosed with a fatigue-causing autoimmune disease.

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.

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.

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.

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. .

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. Ivanovic was playing for the first time since she retired with a left hip injury a week ago at the Indian Wells semifinals.

Two wild cards eliminated seeded women. Garbine Muguruza Blanco of Spain upset No. 9 Vera Zvonareva 6-4, 6-3, and Aleksandra Wozniak of Canada rallied past No. 28 Monica Niculescu 4-6, 6-3, 6-4.

Other seeded women eliminated included No. 11 Francesca Schiavone, No. 18 Angelique Kerber, No. 27 Lucie Safarova, No. 31 Kaia Kanepi and No. 32 Nadia Petrova.

No. 4 Andy Murray, the 2009 champion, beat Alejandro Falla 6-2, 6-3. Also advancing were No. 6 Jo-Wilfried Tsonga, No. 9 Janko Tipsarevic, No. 13 Gilles Simon, No. 16 Kei Nishikori and No. 18 Alexandr Dolgopolov.