Sam Burns shot a second round 61 as he took a one-shot halfway lead at The American Express tournament in California.
Burns’ career-low round leaves him on 17 under par, one shot ahead of fellow American Michael Kim with South Korean’s KH Lee and American amateur Nick Dunlap a further shot back.
A pair of eagles and seven birdies had put Burns in with a chance of breaking 60, but he could only par the last two holes.
Dunlap, who plays at the University of Alabama, fired a 65 at the Stadium Course – one of three being used for the tournament – to follow his opening 64.
A group of six, including American Ryder Cup star Patrick Cantlay and joint overnight leader Alex Noren of Sweden, sit on 14 under par.
Ireland’s Shane Lowry shot a second round of 67 to lie seven shots off the lead.