NEW YORK, NY, USA - Serena Williams rolled through her first two rounds but she was made to work in her third round on Friday, dispatching María José Martínez Sánchez in a tight two-setter, 63 75, to reach the second week.

Williams had her serve broken just once in her first two matches combined, but against Martínez Sánchez she lost it twice, both times during a tight, 56-minute second set. Martínez Sánchez followed her big lefty serve and groundstrokes into the net 36 times during the match, three times as much as Williams; but the No.2 seed handled it beautifully, coming up with enough passes and lobs.

"She was serving really well, especially in the second set. I just kept fighting and every time I had a break point I felt I had a chance to win it," Williams said. "Her style is definitely different. You don't see it that much anymore. I think doubles can definitely help that, because you get to see people at the net. If I hadn't been in the doubles it might have been a much different match."

Get the Wilson equipment the Williams sisters use at Tennis Warehouse!

Next up for Williams will be No.22 seed Daniela Hantuchova, who dispatched wildcard Vania King in one of the first matches of the day, 62 62. Williams is 7-1 lifetime against Hantuchova, including 2-0 here at the US Open.

Another upset rocked the women's draw as No.26 seed Francesca Schiavone rallied past No.8 seed Victoria Azarenka, 46 62 62, to reach the second week of the US Open for the fourth time in her career, her best performance being the quarterfinals in 2003 (she also has two fourth round finishes to her name).

"My tactic was to push her far away, because when she is inside the baseline, it's not easy to play her," Schiavone said. "I was trying to hit deep and open the angles up to be aggressive, go to the net, hit good serves - a lot of things. She's one of the best players so for me it's great I beat her, also how I beat her."

Next up for Schiavone will be No.18 seed Li Na, who beat Maria Kirilenko, 64 62, later in the day. The pair is tied in their head-to-head, 1-1.

Another fourth round match will pit No.7 seed Vera Zvonareva against No.10 seed Flavia Pennetta, after Zvonareva beat No.31 seed Elena Vesnina, 62 64, and Pennetta cruised past Aleksandra Wozniak, 61 61. The pair is tied in their head-to-head, 1-1, but the Russian's win came on clay seven years ago, while the Italian's victory came on the hardcourts of Los Angeles a few weeks ago.

At night, No.3 seed Venus Williams and wildcard Kim Clijsters set up a blockbuster fourth round clash, both winning through in straight sets. Williams recovered from a break down in the second set to beat Magdalena Rybarikova, 62 75, while Clijsters bettered fellow Belgian Kirsten Flipkens, 60 62. Williams and Clijsters have played 10 times before; Williams leads the series, 6-4, but Clijsters won their last two meetings, including right here in 2005, when she rallied from 64 42 down to pull off a 46 75 61 win in the quarterfinals.

Check out Tennis Warehouse for the equipment and apparel the stars of the Sony Ericsson WTA Tour use, at the best prices and delivered to your door!