Andre Agassi's bid for a record sixth San Jose Open title ended in a 6-3, 6-1 quarter-final defeat to Austria's Juergen Melzer on Friday.
Top seed and defending champion Andy Roddick survived a scare against Sweden's Thomas Enqvist to win 3-6, 7-6, 7-5.
Melzer employed a combination of well-timed drop shots and solid groundstrokes to keep Agassi off balance throughout the match.
"I was frustrated. I just never found the range on my shots, especially on my serve," said the 34-year-old Agassi. "He does everything well. He moves well, returns well and has an awkward lefty serve."
Melzer has now won two of their three meetings, having beaten Agassi in Toronto last July.
Agassi, who failed to capitalise on any of his six break point opportunities, was among several players who complained that the balls being used by the tournament were soft and unpredictable.
He showed his frustration when he hit a ball into the stands early in the second set and received a warning from the umpire.
Melzer meets Cyril Saulnier in Saturday's semi-finals after the unseeded Frenchman won 6-2, 6-4 against American Vince Spadea.
Enqvist showed flashes of the player who reached the 1999 Australian Open final but in the end Roddick proved too tough for the 30-year-old.
After losing the first set, the American fought back from 6-0 down to take the second set tiebreaker.
"I was saying to myself, 'I can't believe you're actually blowing this.'" said Roddick. "It was weird. I was up, I was down. It was so back and forth."
Roddick will next face German number three seed Tommy Haas, who beat Max Mirnyi 6-7, 7-6, 6-2 in a two-hour, 10-minute affair.
Mirnyi served for the match at 5-4 in the second set, but his advantage slipped away as Haas overcame 18 aces to take his career record against the Belarussian to 7-0.
