Mickelson got up-and-down for birdie on the par-5 17th and shot 2-under 69 in the steamy sunshine at Riviera, giving him a one-shot lead over Padraig Harrington (70) as he tries to win for the second straight week.
Mickelson was at 13-under 200. He will be in the final group with Harrington and Rich Beem, who delivered the theatrics by making a hole-in-one on the 14th hole that sent him leaping atop the red sports car that he won for his ace. More than getting a new car, Beem suddenly has a chance at his first trophy since the 2002 PGA Championship at Hazeltine.
Beem shot the best round of the day, a 6-under 65, and was only three shots behind.
"I was thinking I won a car, and it was going to cost me an awful lot in the bar later on," Beem said of his ace, a 7-iron from 179 yards that took a high hop and plunged into the cup. "I wasn't thinking about picking up two shots on the field or anything."
Former Nissan Open champion Robert Allenby (68) and Charles Howell III (69) were another shot behind, setting up a typical Sunday off Sunset Boulevard when several players have a chance.
That's due to Mickelson, who one minute looked as though he might run away from the field, and in a span of five holes came charging right back with a couple of missed putts and one wild swing on the par-3 16th.