South Korea’s Sung Kang carded a course record 60 at Monterey Country Club to share the clubhouse lead at the halfway stage of the Pebble Beach Pro-Am.
The world number 306, who shot a level-par 72 on day one at Spyglass in the three-course event, had nine birdies and an eagle to reach 11 under par.
Four-time winner Phil Mickelson was one shot back after five birdies and an eagle in his first 10 holes in a 65.
Justin Rose also played at Monterey and is nine under after a 68.
Sung, who has a best finish of third in his 57 PGA Tour events, holed from eight feet to par the ninth, his final hole.
World number one Jordan Spieth did not drop a shot but made only two birdies in a 69 at Monterey and is three under for the tournament.
Actor Mark Wahlberg, partnering former Masters champion and fellow left-hander Bubba Watson, came within an inch of a hole in one at Monterey in the $7m (£4.8m) PGA Tour event, where celebrities including Clint Eastwood and Justin Timberlake play alongside the world’s top golfers.
Each team plays three rounds on the three different courses, with the leading 25 pro-am teams and 60 professionals returning for the final round at Pebble Beach on Sunday.