Brendan Steele has carded a nine-under-par 63 to earn a one-stroke lead on day one of the Frys.com Open in Napa.
The American enjoyed a bogey-free opening round in California, sinking nine birdies in total to sit one shot clear of Venezeulan Jhonattan Vegas.
Harold Varner III finished Thursday's session on seven-under, one stroke clear of Justin Thomas and DH Lee after the pair shot a first-round 66 apiece.
England's Justin Rose is four off the pace, with the former US Open champion a shot ahead of world number three Rory McIlroy on four-under.