NEW YORK -- Slowed by a right knee injury, eighth-seeded Caroline Wozniacki failed to make it out of the first round of her second straight Grand Slam tournament, falling 6-2, 6-2 to 96th-ranked Irina-Camelia Begu of Romania at the U.S. Open on Tuesday.
Wozniacki quit after the first set of her semifinal match last Friday in New Haven. Four days later, she was ousted from the U.S. Open, finishing the match with tape that trainers applied to her hurting knee during an injury timeout in the second set.
Begu got her first U.S. Open win.
Wozniacki was the top seed at Flushing Meadows the last two years but fell in the semifinals both times.
The 19-year-old Stephens won 6-3, 6-4 against the 2010 French Open champion. Stephens has advanced to at least the third round at three of her last four Grand Slam events. She's ranked a career-best 44th.
"I did everything that my coach asked me to do," Stephens said. "I just really stayed within myself and tried to do my best. For me, that's a big step. If you do everything you want to do in your match, if you accomplish all your goals for today, which I did, I'm pleased, very pleased with myself. That's something that gives you confidence, and that takes you far."
At 3-3 in the second set, Stephens broke Schiavone at love, the game ending on a double-fault. The Italian had seven of those in the match -- five in her first service game.
Stephens had double match point at 5-3 against Schiavone's serve but couldn't convert. Trying to serve out the match, she fought off a break point then clinched the victory with a forehand winner down the line.
"It was an even match, 50-50," said Schiavone, whose ranking has slipped to 26th from a career-best
No. 4 in early 2011. "So whoever is better or does a little more on the important points will win. She had that little extra today."
Preceding Stephens' first-round upset was 32-year-old Venus Williams, playing her first U.S. Open match since she pulled out before the second round in 2011 and revealed she had been diagnosed with an autoimmune disease.
"Honestly, I didn't even understand what I was going through at that time last year. I feel like just this summer I've come to acceptance. Like it takes a long time to come to acceptance, especially when you're an athlete. You see yourself as this healthy person that nothing can defeat you," Williams said. "So it takes a while before you can kind of see yourself as someone with flaws and chips [sic] in the armor. Now that I have come to accept it, it helps me a lot in how I need to prepare for my matches, the mindset I need to come into it."
After a shaky start, dropping the first two games -- and even seven points in a row in one stretch -- Williams used her own powerful serve to right herself and beat Bethanie Mattek-Sands of the U.S. 6-3, 6-1. One serve at 124 mph jammed Mattek-Sands' left index finger, shoving it into a racket string so hard she needed attention from a trainer.
"She was crushing her serves," Mattek-Sands said. "I don't think anyone's returning those, so I'm not going to beat myself up too much."
Venus Williams won the 2000 and 2001 U.S. Opens, two of her seven career Grand Slam titles. That's half as many as her younger sister, Serena, who began her bid for No. 15 with a 6-1, 6-1 victory over 75th-ranked CoCo Vandeweghe in Tuesday's last match in Ashe, yet another all-American affair.
"Venus is amazing. She's the ultimate role model for me," Serena Williams said. "She's the ultimate fighter and champion -- everything she's gone through and is going through. I have no excuses any more. She makes me a better person."
The younger Williams won the U.S. Open in 1999, 2002 and 2008, and is among the favorites in 2012 considering the way she dominated the competition recently while winning Wimbledon and a gold medal at the London Olympics.
"We need more American champions here to hold up these amazing trophies," Serena Williams said.
Earlier, second-seeded Agnieszka Radwanska needed just 54 minutes to advance to the second round. Radwanska beat Nina Bratchikova of Russia 6-1, 6-1 on Tuesday to start play at Arthur Ashe Stadium. The 91st-ranked Bratchikova had 28 unforced errors to only six for Radwanska.
The Polish star is coming off her first Grand Slam final, a three-set loss to Serena Williams at Wimbledon. She has never made it past the fourth round at Flushing Meadows. Radwanska retired from her previous match with a sore shoulder while trailing qualifier Olga Govortsova in her opener at New Haven.
Bratchikova was making her U.S. Open debut, but reached the third round at the Australian and French this year.
Kerber, seeded sixth, moved on easily, beating Britain's Anne Keothavong 6-2, 6-0 in 54 minutes Tuesday. Keothavong, ranked 81st, had 25 unforced errors to Kerber's nine.
Ranked outside the top 100 at the start of 2010, Kerber is now a career-best sixth. She reached her first Grand Slam semifinal at Flushing Meadows last year.
Information from The Associated Press is included in this report.