Isner's serve allowed him to get by Spain's Robredo 7-6 (7-5), 3-6, 6-4 in a tight quarterfinals match. The American got 73 percent of his first serves in and won more than three-quarters of the points when that happened. Robredo landed just more than half of his first serves and had seven double faults in the match.
Sixth-seeded Austrian Jurgen Melzer was also ousted, losing 6-2, 7-5 to France's Arnaud Clement, who upset second-seeded David Ferrer in the second round. Clement didn't drop serve and broke Melzer's serve four times.
Also Thursday, fifth-seeded German Philipp Kohlschreiber made short work of Frenchman Marc Gicquel, winning 6-3, 6-1, and No. 8-seeded Spaniard Albert Montanes stopped Swiss Michael Lammer 6-3, 6-3.
Montanes draws Isner in Friday's semifinals. Isner beat Montanes last January in this event in the only career meeting between the two. Clement and Kohlschreiber, the other semifinal pairing, have also met once before. Kohlschreiber won in straight sets when the two played in 2007.