WIMBLEDON, England — 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 today and reach the third round of Wimbledon.

For the second straight match, the four-time champion faced a modest British opponent in the 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.

In men’s play, second-seeded Rafael Nadal rallied to beat 19-year-old Latvian star Ernests Gulbis 5-7, 6-2, 7-6 (2), 6-3 on Court 1 as he continued his bid of becoming the first player since Bjorn Borg in 1980 to win the French Open and Wimbledon in the same year.

Williams took 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.

There was one tense moment in the second set when, with the two players across the net from each other, Keothavong ripped a backhand that hit Williams full force in the neck area.

“It hurt,” Williams said. “This is tennis. You’ve got to be ready for whatever. I’ve hit some people, too. That’s just how it goes sometimes. I don’t think she was aiming for me.”

Nadal, a two-time Wimbledon runner-up who swept to his fourth straight French Open title without dropping a set, lost the first set against the talented Gulbis — a quarterfinalist at the French — when he was broken in the 12th game.

But Nadal sailed through the next set, took the third in a tiebreaker and got the decisive break for 5-3 in the fourth.

After hitting a forehand winner on match point, Nadal let out a shout, dropped into a crouch and pumped his left arm four times — an unusually ebullient celebration after a second-round match, but indicative of the danger that Gulbis had posed.

“I’m very happy,” Nadal said. “I knew I was going to have a very tough match against a very aggressive player. I feel I improved during the match.”

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 this 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.”