WIMBLEDON, England — Defending champion Venus Williams moved a step closer to her fifth Wimbledon singles title today, beating Tamarine Tanasugarn in straight sets to reach the semifinals and close in on another potential championship matchup with sister Serena.

Ms. Williams downed 31-year-old Ms.Tanasugarn 6-4, 6-3 on Court 1 to extend her career record over the Thai player to 7-0. The seventh-seeded American, who hasn’t dropped a set all tournament, was limping slightly at the end of the match with a hamstring problem.

“It feels a little bit tight,” she said. “I’ll be fine. I’m a big girl. I can deal with it.”

Two-time champion Serena Williams, seeded No. 6, was scheduled on Centre Court later against 19-year-old Agnieszka Radwanska of Poland.

The Williams sisters are in opposite halves of the draw and could meet in Saturday’s final. The two have been twice before in the Wimbledon final, with Serena winning both in 2003 and ’04.

“That would be amazing if we both were in the final,” Venus Williams said. “I have to take it one more step and keep playing power tennis.”

Today’s other matchups were No. 5 Elena Dementieva vs. Nadia Petrova, and No. 18 Nicole Vaidisova vs. China’s Zheng Jie.

The 60th-ranked Tanasugarn, playing in her first Grand Slam quarterfinal, pushed Ms. Williams as hard as she could but didn’t have enough to cope with her power game.

Williams served eight aces and had one serve at 127 mph, while Tanasugarn had no aces and had an average first-serve speed of just 90 mph.

Ms. Tanasugarn fashioned 10 break points, but converted only once. The key game was the sixth of the first set, when Ms. Williams saved six break points — mostly on Ms. Tanasugarn errors — and finished with a 126 mph service winner to hold for 4-2.

Ms. Dementieva was the highest seeded player left in the women’s draw after yesterday’s upsets of No. 2 Jelena Jankovic (by Ms. Tanasugarn) and No. 4 Svetlana Kuznetsova (by Ms. Radwanska).

With top-seeded Ana Ivanovic and No. 3 Maria Sharapova eliminated last week, none of the top four seeded women reached the quarterfinals — the first time that’s happened at Wimbledon and also the first time at any Grand Slam tournament in the 40-year history of the Open era.

The men’s quarterfinals are set for tomorrow, with five-time champion Roger Federer and two-time runner-up Rafael Nadal seemingly headed toward a third straight championship showdown.

Mr. Federer has cruised into the quarterfinals without dropping a set and extended his winning streak on grass to 63 matches and 38 in a row at Wimbledon. He next faces Mario Ancic, the last player to beat Mr. Federer on grass — in the first round of Wimbledon in 2002.

No. 2 Mr. Nadal required treatment behind his right knee after slipping in the opening set of his fourth-round win over Mikhail Youzhny. He next faces Andy Murray, who has reached his first Grand Slam quarterfinal and is carrying the hopes of the nation for Britain’s first male Wimbledon champion in 72 years.

The other men’s quarterfinal matchups are Marat Safin vs. Feliciano Lopez, and Rainer Schuettler vs. Arnaud Clement.