Phil Mickelson followed his career-best 60 with a four-under 68 to remain three shots clear nearing the end of day two at the Desert Classic in California.
The 48-year-old, who led overnight after his 10 birdies and an eagle at La Quinta, had a double bogey in round two at the Nicklaus Tournament Course.
But he birdied four of the final five holes for a 16-under halfway total.
Also at the Nicklaus course, world number one Justin Rose shot a second successive 68 and is eight shots back.
Russell Knox had seven birdies in a 65 at La Quinta to reach five under but fellow Scot Martin Laird is one further back after finishing with two double bogeys to return a 75 after his opening 65.
On Saturday, Mickelson and Rose will play the third venue of the event, the Stadium Course, which also hosts Sunday’s final round.
Mickelson began his round at the 10th and birdied his second and third holes.
The double bogey came at his ninth hole but he finished in style with a superbly controlled pitch spinning back to five feet at the last for his sixth birdie of the day.
Asked to compare his second round with his 60, the American said: “I actually felt better striking it but I didn’t putt very well relative to yesterday where I was making everything.”
During his round, the left-hander was about to take a drop using the long-standing method from shoulder height, but a rules official was on hand to remind him that – as from 1 January – it should be done from the knee, and no penalty resulted.
The last time Mickelson held the lead at the halfway stage of a tournament was at the 2016 Open when he went on to finish runner-up to Henrik Stenson.
“This has been a fun, if not surprising, start for me and it’s fun to get in the mix,” he added.