Sampras, the top seed, who has twice won this event, could not find his rhythm. He had complained of flu-like symptoms on Wednesday, but made no excuses after Thursday night's defeat.
"The spring was there in my legs," he said. "It was just an off day against someone who has been playing good."
Haarhuis, a finalist in Philadelphia last year, found himself able to break Sampras' serve with unusual ease. At the start of the second set, he broke the American twice in succession to lead 3-0 and although Sampras managed to retrieve one of the breaks he could not reverse the momentum of the match.
The upset may assist Andre Agassi in his campaign to take over at No 1. Agassi, the second seed, and Michael Chang, the third, are the only seeded players left in the event. Chang reached the quarter-finals with a 7-6, 7-5 victory over Jim Grabb.Reuse content