Phil Mickelson fired a career best-equalling 60 to take the early lead as he made his 2019 debut at the Desert Classic in California.
The five-time major champion shot 10 birdies and an eagle during a blemish-free day on the par-72 course at La Quinta.
The 48-year-old, with two previous 60s at the Waste Management Phoenix Open in 2005 and 2013, hit his best ever career round to par and put himself top of the pack on 12 under.
The American narrowly missed shooting his first ever round of 59, needing two birdies on the last two holes to reach the feat. He made par on the 17th before holing a birdie from 10 feet on the 18th.
In a video posted on the PGA Tour's Twitter page, Mickelson said after his round: "I really didn't think this was going to be a day when I would go low. I came in with very low expectations.
"I haven't had a lot of time to practise and prepare. I felt like all areas were OK, but you never really know until you get out and play and compete.
"The good thing was is I made a couple of bad swings and I got away with them."
The tournament is held over three courses – La Quinta, the Nicklaus Tournament Course and PGA West's Stadium Course.
Mickelson's closest challenger, compatriot Adam Long, is three shots off the pace after carding a nine-under 63 on the Nicklaus course.
Englishman Justin Rose, the world number one, finished tied for 31st on four-under after an opening round 68 at La Quinta, while Scot Martin Laird sits in joint fourth on seven-under after mixing two bogeys and nine birdies for a round of 65 at Nicklaus.
