Gambill, an American ranked 71st in the world, won the match as Philippoussis double-faulted, by smashing a hard second serve well out. Gambill also had a match point moments earlier at 30-40, but then the Australian produced a typical ace to get himself out of trouble.
Gambill will play the veteran Magnus Gustafsson in the semi-finals today. Gustafsson also was forced to a decisive set, outlasting Jonas Bjorkman 6-4, 0-6, 7-5 in an all-Swedish quarter-final.
Venus Williams, the No 3 seed, started slowly, then took command to overwhelm the 16-year-old Czech qualifier Daniela Bedanova 6-4, 6-1 to advance to the quarter-finals of the Advanta Championships in Philadelphia.
The 19-year-old Williams next plays Sandrine Testud of France, who upset seventh seed Dominique Van Roost of Belgium 6-4, 6-4 in their second-round match.
Trailing 4-2 in the first set, Williams came alive to win 10 of the next 11 games, breaking Bedanova's serve five straight times in the process. "I just dropped my serve a couple of times," said Williams.
The fourth seed, Nathalie Tauziat, of France, rallied from a 5-2 third- set deficit to beat America's Jennifer Capriati 6-4, 3-6, 7-5. Amy Frazier, also from America, kept alive her slim hopes of qualifying for the Chase Championships by beating Justine Henin of Belgium 7-5 6-4.