Brandon Hagy didn't find out he was in the field at the PGA's The American Express until three days before it started. The Californian then left no doubt he belonged.

Hagy took the first-round lead on Thursday with an eight-under 64 on the Nicklaus course, staying one shot ahead of South Korea's Byeong Hun An in the opening tournament of the PGA Tour's West Coast swing.

Hagy racked up 10 birdies, including three streaks of three in a row. He capped the round with a calm birdie putt on the 18th.

"I felt like I saw the greens really well, and the last couple of days, I felt like I was putting really well," Hagy said. "Once I started to read them well, I knew if I could just keep doing what I was doing, I'd have a good shot to do something."

Hagy was only added to the field Monday when second-ranked Jon Rahm abruptly dropped out without giving a reason to tour officials. Rahm later said he incurred a minor injury in the gym, but the Spaniard plans to play at Torrey Pines next week.

Hagy might have made it into the field anyway after additional dropouts, but he was still grateful to Rahm, his friend and a collegiate opponent when Rahm played at Arizona State: "I'll have to send him a gift or something."

An made seven birdies without a bogey in his 65. Max Homa, Martin Laird and Si Woo Kim shot 66.

Australians Aaron Baddeley and Cameron Davis both shot four-under 68 at the Nicklaus Tournament Course to be four behind Hagy in a tie for 15th.

Baddeley blitzed the front nine with five birdies to turn on 31, but he came home in 37 after a double bogey on the par-five 11th and bogey on par-three 17 accompanied birdies on 12 and 15.

Davis mixed his card with six birdies and two bogeys.

Compatriot Matt Jones, who played at the Stadium Course, was two shots further adrift with Rhein Gibson.

Tournament host Phil Mickelson shot a 74 with swing coach Andrew Getson serving as his caddie.

The 50-year-old left-hander indicated before the start if his performances weren't up to scratch he would consider playing in more events on the Champions Tour.