Brandon Hagy carded an eight-under 64 to take a one-stroke lead after the first round of The American Express on Thursday.
Hagy made 10 birdies and two bogeys during his first round on the PGA West Nicklaus Tournament Course in California.
The American sits a shot clear of Byeong Hun An, who fired a 65 on the same course.
Hagy is in the lead through 18 holes for the second time in his PGA Tour career and first since the 2017 Canadian Open.
Kim Si-woo produced the best round of players who started on the PGA West Stadium Course, his 66 – which featured an eagle at the fifth hole – seeing him sit tie for third.
Martin Laird and Max Homa are alongside Kim, two shots adrift of Hagy.
A group of nine players opened with 67s, including Charl Schwartzel and Alex Noren.
Defending champion Andrew Landry, meanwhile, carded a two-under 70 on the Stadium Course to be back in a tie for 61st.