Junqueira is trying to close the gap on Champ Car series leader Paul Tracy and defend his title.
Junqueira sped around the tight, 1.647-mile street course at 96.507 miles per hour, giving himself an opportunity to end a two-race scoreless streak that cost him his championship lead.
The Brazilian earned a bonus point for taking the provisional pole, giving him 165 this season, 27 behind Tracy with five races remaining.
Tracy was fourth Friday at 95.546 mph. Mexico's Michel Jourdain Jr., who won last week in Montreal, is third in the standings, 35 points off the pace.