Isner needed just 56 minutes to dispose of Bernard Tomic 6-3, 6-2, setting up a semifinal duel with Anderson.
Anderson survived a key second-set tiebreak to defeat Roddick 2-6, 7-6 (11-9), 6-4. Roddick won two matches in a tournament for the first time this year, but could not keep that run going.
The other semifinal will send Dudi Sela against Marinko Matosevic in a meeting of unseeded players.
Sela, in search of his first ATP Tour championship, surprised No. 5 seed Philipp Kohlschreiber 6-4, 6-7 (4-7), 6-0. Matosevic was a 3-6, 6-1, 6-1 winner over Ernest Gulbis, who had been trying for his second title in three years at this event.