STUTTGART, Germany — Venus Williams reached the Porsche Grand Prix semifinals, defeating third-seeded Dinara Safina 6-4, 6-2 today in the first match between the players.

In another quarterfinal, Olympic champion Elena Dementieva, seeded fourth, lost to 19-year-old Victoria Azarenka of Belarus 7-6 (6), 3-6, 6-1.

Azarenka, grunting loudly on almost every point, will next play another Russian in Nadia Petrova. Petrova, the 2006 Stuttgart champion, beat Li Na of China 6-2, 6-3.

Williams will next play Jelena Jankovic or Vera Zvonareva. In defeating Safina, Williams eliminated an opponent who has won four titles this year and risen to a career-high No. 3 ranking.

"I had to work very hard because she played at 200 percent," Williams said. "It's very exciting to win against a player who has had such a great year."

Williams' only title this year was Wimbledon. She is playing in Stuttgart for the first time in 10 years and eyeing the red Porsche 911 sports car given to the champion.

"I love fast cars," Williams said. "I really hope to try it as the winner."

Williams broke for a 5-4 lead and hit a service winner to take the first set. She jumped ahead 4-0 in the second. She also used her big reach to win points at the net.

"I feel more comfortable at the net now," Williams said. "I have a volley."

Azarenka is ranked No. 17 and has yet to win on the tour. Dementieva has 10 career titles and has returned to her highest ranking of No. 4, which she reached three years ago.

Dementieva's many errors in the third set allowed Azarenka to race to a 5-0 lead. Dementieva saved three match points to win her first game, but then committed two straight errors and netted a backhand to lose after nearly three hours.

"We had many long rallies," Azarenka said. "I tried to fight for every ball. I played very well."