Viktor Hovland birdied five of his first nine holes on Friday at the Farmers Insurance Open as the Norwegian took the lead in California.
Starting on the back nine, the 23-year-old reached the turn at four under for the round after dropping a shot on the 15th, then picked up three more shots en route to a 65.
He moved up 47 places to sit at the top of a stacked leaderboard at nine under going into the weekend.
Six players, including joint overnight leader Patrick Reed, are tied for second, with the American only able to card a par 72.
A group of three are a shot further back, including Robby Shelton who went round in 64, while the other overnight leader Alex Noren fell 10 places after his round of 74.
Rory McIlroy followed his 68 with a 71 and is tied for 14th on five under, while Si Woo Kim, Martin Laird and Harris English were among those to miss the cut at one under.