After a marathon first set lasting longer than many of her matches, defending champion Venus Williams overcame another erratic performance and pulled away to beat Britain's Anne Keothavong 7-5, 6-2 and reach the third round of Wimbledon.

For the second straight match, the four-time champion faced a modest British opponent in Thursday's opening contest on Centre Court and was tested to the limit in the first set. The pattern and result were almost identical from her 7-6 (5), 6-1 win over Naomi Cavaday on Tuesday.

The first set alone lasted 1 hour, 9 minutes as Williams struggled to take command against a determined 92nd-ranked player who came into the tournament with only one win at Wimbledon in seven attempts.

"I lost a little bit of focus (in the first set) but got it back thankfully," the American said.

Williams jumped out to a quick 2-0 lead before lapsing into a flurry of errors that turned the set into a battle of attrition, with the fourth and fifth games taking more than a half an hour.

Keothavong held for 2-2 after going to seven deuces, and then Williams saved eight break points in a game that went to 10 deuces, closing it out with a 124 mph service winner.

They went back and forth on serve until Williams broke for the set in the 12th game, hitting a deep backhand return that forced a forehand mistake by Keothavong. Williams broke for 3-1 in the second set with another deep serve return and cruised the rest of the way to close it out in 1 hour, 44 minutes.

Williams finished with six double faults and 26 unforced errors, 10 more than Keothavong.

"I don't think she liked the way she played," said Williams' mother and coach, Oracene Price. "Well, she wasn't doing the things she should do and knows she should do. It's good it's happening early on. She knows she's going to have to tighten it up."

The women's field lost one former champion, 1999 winner Lindsay Davenport, when she withdrew with a right knee injury before her second-round match against Argentina's Gisela Dulko.

The 32-year-old Davenport had limped past Renata Voracova in three sets in her opening match on Tuesday and decided she wasn't fit to play after practicing Thursday morning.

"It's just really inflamed and painful," she said. "I rested all day yesterday and did treatment. After warming up, I felt like I was 25, maybe 30 percent. That's not good enough for a second-round match."

It was Davenport's 13th visit to Wimbledon and could be her last.

Asked if she would be back next year, she paused and said: "I guess not. It did come into my mind today that it is my last chance to play here."

In men's play, second-seeded Rafael Nadal was on Court 1 against talented 19-year-old Latvian Ernests Gulbis, and two-time runner-up Andy Roddick was to face Janko Tipsarevic of Serbia.

In other women's matches, 2004 champion Maria Sharapova was paired against fellow Russian Alla Kudryavtseva and No. 2-seeded Jelena Jankovic faced Spanish wild card Carla Suarez Navarro.