Wozniacki saved three consecutive set points in the tiebreak but finally prevailed when her opponent sent a forehand volley into the net on her first match point.
"I started off really well, and everything was going the way I
wanted it to," said Wozniacki, who is still chasing her first
Grand Slam title. "Second set, she played better. I started to
play a little bit worse."
Wozniacki again played with a bandage on her left thigh and never looked comfortable on court, making 24 unforced errors.
The Dane had her left leg treated during her victory in the final of a clay-court tournament in Brussels last weekend.
"It's preventative," Wozniacki said. "I felt it a little bit
last week and I didn't want to make it worse, so I just do it for
protection, because, I mean, there is no reason to make it worse."
Defending champion Francesca Schiavone cruised past Vesna Dolonts of Russia 6-1, 6-2, winning 11 games in a row. Samantha Stosur, last year's runner-up, advanced to the third round by beating Simona Halep 6-0, 6-2. The eighth-seeded Stosur had 21 winners and broke her opponent six times before she served out the match at love.
Sabine Lisicki of Germany was bothered by dizziness and muscle cramps and was carried off on a stretcher, sobbing, after wasting a match point and a 5-2 lead in the third set of a 4-6, 7-5, 7-5 loss to two-time Grand Slam finalist Vera Zvonareva of Russia. Lisicki was treated by a trainer -- including a blood-pressure check -- during changeovers.
"I hope she feels OK," the No. 3-seeded Zvonareva said. "No matter what, I had to keep doing my job."
The 34th-ranked Mattek-Sands lost in the first or second round at 19 of her previous 20 Grand Slam tournaments. The exception: She reached the fourth round at Wimbledon in 2008.
"It was a tough match, but third set, I came out and wanted to be aggressive," the 34th-ranked Mattek-Sands said. "I noticed she was struggling when I hit hard and deep, so that was my goal -- to do that more."
She also pushed forward whenever there were openings and won 14 of 21 the points at the net.
"I guess she was just not missing, and I had few unlucky shots that kind of decided the ... third set," she said. "Few points here; few points there."
According to the WTA, Wednesday's result means Mattek-Sands is projected to pass Venus in the rankings released after the French Open. That would make Mattek-Sands the No. 2 American woman, behind only Serena.
Serena is 17th this week, and Venus is 30th. Both withdrew from the French Open because of health issues and injuries. Depending on how long the sisters stay sidelined, and how Mattek-Sands performs, she could be the top American sometime this summer.
"It would be huge," Mattek-Sands said. "But when you think of American tennis, you think of No. 1 in the world. So I don't think I'm officially the No. 1 player until I can get top 10. That's my goal."
Another goal, she said this week, is to be among the 32 women who get seeded at a Grand Slam tournament.
No. 10 Jelena Jankovic of Serbia, No. 11 Marion Bartoli of France, No. 13 Svetlana Kuznetsova of Russia, No. 14 Anastasia Pavlyuchenkova of Russia, No. 17 Julia Goerges of Germany, No. 28 Daniela Hantuchova of Slovakia and No. 29 Peng Shuai of China also won, while Gisela Dulko of Argentina eliminated No. 32 Tsvetana Pironkova of Bulgaria.
Information from The Associated Press was used in this report.