Maurice Greene no longer looks like the clear-cut favorite for the 100-meter gold medal in Athens. Asafa Powell, a 21-year-old Jamaican, has joined him as the man to beat.

Powell, undefeated this season, clocked 9.91 seconds -- the fastest time ever on British soil -- to win the 100 at last night's London Grand Prix meet.

Greene, the 2000 Olympic gold medalist, three-time world champion and former world record holder, finished second in 9.97. Another American, Leonard Scott, was third in 10.04.

Also yesterday, Russia's Yelena Isinbayeva broke the pole vault world record for the third time in five weeks, clearing 16 feet3/4 inch at Crystal Palace.

"It doesn't matter what happened here," Greene said. "Everything matters what happens at the Games."

Alan Webb clocked 3 minutes 50.73 seconds in the mile, beating his previous best of 3:50.85, but that was only good enough for fourth in the 52nd running of the Emsley Carr mile.

* TENNIS: Top-ranked Roger Federer stretched his winning streak to 21 matches by beating Fabrice Santoro, 7-5, 6-4, to reach the Tennis Masters Canada semifinals in Toronto.

Federer, who has won three straight tournaments since losing in the French Open's third round, next faces Thomas Johansson or Joachim Johansson.

Later last night, defending champion Andy Roddick was to play Czech qualifier Jan Hernych, and Jurgen Melzer faced Nicolas Kiefer. . . .

Lindsay Davenport defeated Ai Sugiyama, 6-1, 6-3, at the Acura Classic in Carlsbad, Calif., to advance to the semifinals of her third consecutive hard-court tournament. Serena Williams withdrew yesterday because of swelling in her left knee. Venus Williams withdrew Tuesday because of a sprained right wrist.

Vera Zvonareva advanced when Serena Williams pulled out. She will play the winner of a late match between French Open champion Anastasia Myskina and 17-year-old Wimbledon champion Maria Sharapova. Elena Dementieva defeated Amy Frazier, 6-2, 6-1. She will play Davenport in today's semifinals.

* SWIMMING: The U.S. Anti-Doping Agency announced yesterday that Rachael Burke of Bethesda tested positive for boldione, a prohibited substance, in an out-of-competition test on May 1. Burke, 21, accepted and began serving the provisional suspension on Thursday.

-- From News Services

and Staff Reports