Haas, playing just his ninth Champions tournament of the season, won for the second time in three weeks. He was victorious earlier this month at the Greater Hickory Classic.
Tom Purtzer fired the low round of the day as he carded an 8-under 63 to take second place at 12-under-par 201. Mark James ended one stroke further back at minus-11.
Dana Quigley, who led most of the round, bogeyed each of the last three holes to slide into a tie for fourth place at 10-under-par 203 after a round of 1-over 72. He was tied there by R.W. Eaks (66).
Eaks was rushed to the hospital shortly after completing his round. According to The Golf Channel, he had his blood pressure tested several times during the round and it was high.
Eaks' blood pressure was checked again after the round and was taken to a local hospital for tests immediately after signing his scorecard. Despite the health issues, Eaks earned enough money to move into the 30th spot in the money list, gaining him a spot in next week's Charles Schwab Cup Championship.
Haas started steady at Oak Hills Country Club as he parred his first four holes. Haas then birdied the par-5 fifth for the third straight round.
The nine-time winner on the PGA Tour came right back with a birdie on No. 6 to tie Quigley in the lead at 11 under. Haas parred seven, then birdied the eighth. He made it two straight, and again tied Quigley in the lead at minus-13, as he birdied the ninth.
Haas parred the first five holes around the turn to stay at minus-13. He then got up and down for birdie from just short of the 15th green to move one shot clear of Quigley.
The 51-year-old closed with three straight pars as Quigley, his playing partner, collapsed late. Haas won for the third time at Oak Hills, as he won the Texas Open here in 1982 and 1993.
'I'll tell you, I was leaking oil coming in there. That was really a hard last hole,' said Haas, who collected $232,500 for the win. 'It was 202 yards into the wind and I was kind of between clubs. It's hard to play when all you have to do is bogey. I was trying to screw that up, but I managed to make the putt.'
Quigley, who entered the day 2-2 in playoffs this year alone, opened with a birdie on the second, before tripping to a bogey at three. He drained back-to- back birdies from the seventh to briefly move one clear of Haas.
After Haas tied him on nine, Quigley regained the lead with a birdie on 11. However, he stumbled to a three-putt bogey on the next to fall back to 13 under.
Quigley fell apart down the stretch. He found a fairway bunker off the 16th tee. He nearly shanked his second shot out of the bunker as the shot came up well short and right of the putting surface.
The leader of the money list and the Charles Schwab Cup points list then hit a tree off the tee on 17. Quigley knocked his second down the fairway, but was unable to save par. He missed the green right in a bunker on 18 and could not save par again to finish four back.
Despite the late collapse, Quigley has two wins and five second-place finishes among his 14 top-10s this year. However, he has led four times entering the final round this year and has not won one of those four events.
'I got a bad break on 16. My shot was 6 inches from probably a pretty good shot,' Quigley said. 'I got a little flat after the bogey at 16. I knew (Haas) wasn't going to give it up. I didn't capitalize when I needed to.'
Purtzer ran off four consecutive birdies from the third to jump to 8 under. He eagled the par-5 10th for the second day in a row. Purtzer birdied the 12th, but dropped a shot on 14. He closed with birdies on 15 and 17 to get within two of Haas.
Tom Kite, Gil Morgan and Jerry Pate shared sixth place at 9-under-par 204. Dan Pohl and Bruce Fleisher were one stroke further back at minus-8. Mark McNulty, the 2004 champion, was among three players tied for 11th place at 7-under-par 206.