Dave Eichelberger and Bruce Lietzke are knotted in fourth place at minus-5.
Perhaps the biggest story to come out of Friday's opening round was that Arnold Palmer withdrew after four holes due to back soreness. He continued his round with Lee Trevino and John Mahaffey although did not keep a score.
After the round, Palmer acknowledged that he believed this was his final competitive round.
'To stand out there and not be able to make something happen is very traumatic in my mind,' said Palmer. 'When the people want to see a good shot and you can't give them that shot, that's when it's time.'
Haas wasted little time in breaking into red numbers as he rolled in an 8-foot birdie putt at the par-5 second. He ran home a 20-foot birdie putt at the fifth, but was not done going low on his front nine.
At the par-4 seventh, Haas knocked his approach to 3 feet. He converted that birdie putt, then kicked in another short birdie putt at eight to make the turn at 4-under-par 32.
On the second nine, Haas drained a 5-foot birdie putt to reach 5 under par for the championship. He hit a hybrid-club inside 2 feet at the par-3 15th to set up another birdie, then closed his round with a 25-foot birdie putt at the 17th.
'I really played well,' acknowledged Haas, who is second on the Champions Tour money list. 'I've been swinging well and I felt confident coming into the week.'
Purtzer began on the second nine Friday and tallied a pair of birdies from inside 10 feet at the 12th and 13th holes. He hit a 3-iron 20 feet from the flag at 15 and holed the birdie putt.
He continued his fine form on his second nine. At the first, Purtzer sank a 10-foot birdie putt, then pulled the wrong club out of the bag at three and made bogey.
Purtzer overcame the dropped shot at the third with a 20-foot birdie putt at four. He added birdies at six and eight to join Haas and Mast atop the leaderboard.
'I've been playing good the last three months,' said Purtzer, who has won one event each of the last three years. 'I made key putts during the round. I think I've got the right mental approach.'
Mast also started on the back nine and played brilliantly over his first nine holes. He notched five birdies on his opening nine and reached 7 under par for the championship with an eagle at the par-5 second.
Things went downhill quickly. Mast bogeyed the third, but reclaimed the lost stroke with a birdie at the par-5 sixth. Mast double-bogeyed seven, but birdies at his final two holes gave him a share of the lead.
Curtis Strange, Dana Quigley, last week's SAS Championship winner Tom Jenkins, Bill Rogers and two-time winner this season Brad Bryant share sixth place at 4-under-par 68.
Defending champion Mark McNulty shot a 1-under-par 71 and is part of a group tied for 26th place.