It was Sharapova's first match in six weeks since her defeat in the third round of the U.S. Open. The second-seeded Russian, who was sidelined because of a lingering shoulder injury, has never made it past the second round in the event.

Sharapova went ahead 5-3 and 40-love in the first set but lost 10 consecutive points and allowed Azarenka to force a tiebreaker.

"Maybe I was too self-assured at 5-3," Sharapova said. "But it all went downhill after."

In the men's event, second-seeded Mikhail Youzhny and third-seeded Andy Murray advanced to the second round. Youzhny beat Ernests Gulbis 6-3, 6-2, and Murray defeated Evgeny Korolev 6-2, 6-4. Paul-Henri Mathieu defeated qualifier Igor Kunitsyn 6-3, 6-4.

Against Sharapova, the 18-year-old Belarussian saved three more set points in the tiebreaker before taking the set. Azarenka then broke the two-time Grand Slam champion twice in the second set for her first win over the fourth-ranked player.

"I tried to stay concentrated and believed I could win the match," said Azarenka, ranked 32nd, who was a runner-up in her first WTA Tour final in Tashkent last weekend.

Vera Zvonareva upset Amelie Mauresmo 4-6, 6-3, 6-4 in the first round.

"She took the chances she got," Mauresmo said. "She began pretty bad and I was doing well, but then it went the other way."

In the second round, top-seeded Svetlana Kuznetsova was the only seeded woman to win today after she rallied to beat Gisela Dulko 3-6, 6-2, 6-4.

Vera Dushevina upset Wimbledon finalist Marion Bartoli 2-6, 6-0, 6-4, and Elena Dementieva defeated seventh-seeded Patty Schnyder 6-3, 6-4. Dementieva will face Azarenka in the quarterfinals.

Kuznetsova and Dulko struggled to hold serve.

"It seems to me that I was playing against myself, surface and balls today, not against her," said Kuznetsova, who had four aces and 55 unforced errors. "I played bad but nevertheless won the match."

Mauresmo, who lost to Zvonareva for the first time in eight matches, took a 4-0 lead before winning the first set.

But the Russian broke the former top-ranked Frenchwoman in the second game of the second set and again in the opening game of the third. Mauresmo broke back at 4-4 but made three unforced errors on her serve in the next game, letting the 24th-ranked Russian serve for the match.