LOS ANGELES, July 24 AAP-AFP - Mark Philippoussis will make his return return from injury as a wildcard entry in the ATP Mercedes Benz Cup here tomorrow.

Philippoussis is playing for the first time since missing Australia's Davis Cup semi-final against Brazil in Brisbane just over a week ago because of knee soreness and tiredness after Wimbledon.

Second seed Philippoussis will go into the tournament as the top ranked player after No.1 seed Andre Agassi withdrew today, still troubled by the back injury which kept him out of the US team's Davis Cup 5-0 semi-final loss to Spain in at the weekend.

Philippoussis opens his account against American journeyman Vince Spadea tomorrow as the US hardcourt season begins its build up to the US Open from August 28-September 10.

Reigning US and Australian Open champion Agassi was injured when his his car was struck from behind as he was bound for his Las Vegas home after a flight from England following his Wimbledon semi-final loss to Pete Sampras.

The 30-year-old Agassi has suffered back spasms following the accident.

"I suffered a spasm in the middle of the night that made it feel sore and stiffer than it needed to be," said Agassi at the tournament venue today.

"It's just not ready yet."

"This has been incredibly disappointing to start the summer," Agassi said. "First it was the Davis Cup and now it's creeping into the summer. My first priority is to get 100 percent healthy."

Chile's Marcelo Rios, who won his first title of the year Sunday in Croatia, is seeded third.

The only two seeds in action today's opening day of play won in straight sets. South Africa's Wayne Ferreira, seeded fifth, beat France's Anthony Dupuis 6-2 6-2, and eighth-seeded Frenchman Arnaud Clement beat South Africa's Marcus Ondruska 6-3 6-2.

Agassi won here in 1998, beating Britain's Tim Henman in the final. Sampras beat Agassi in two tie breaker sets in last year's final but decided not to defend the crown at the first US hardcourt tuneup on the road to the US Open.

Chile's Marcelo Rios, who won his first title of the year Sunday in Croatia, is seeded third.