Haas shot a 10-under 60 to move five shots in front midway through the Charles Schwab Cup Championship. That gave him a 14-under total of 126 for two trips around the Desert Mountain Cochise Course.
Lehman moved into second place at 131 after firing a 63. He is three shots and three places in the standings in front of Bernhard Langer, who came into the final event on the Champions Tour leading Lehman in the race for the Charles Schwab Cup.
The points race winner earns $1 million.
If both Lehman and Langer finish in the positions they now occupy in the standings, Lehman would pass Langer and become the first player to win the Charles Schwab Cup two years in a row.
Haas birdied seven holes on the back nine to shoot a 28. He got it up and down from a greenside bunker on the par-5 18th for a closing birdie.
"I didn't think that bunker was a bad place to be, but if you are trying to make eagle (to shoot a 59) it is a bad place," Haas said.