Jordan Spieth and Sergio Garcia set the pace on day one of the Charles Schwab Challenge but US PGA Championship winner Phil Mickelson stole the headlines, despite his poor start.

Spieth and Garcia both carded bogey-free seven-under-par 63s to share the two-stroke lead following the first round of the PGA Tour tournament on Thursday.

Former world number one Spieth made a hot start at Colonial Country Club, where the three-time major champion tallied seven birdies, including three in a row to close out his round.

Spieth's flawless performance resulted in the 2016 champion's lowest score in 33 rounds at the tournament.

Garcia – the 2017 Masters winner – finished with an eagle and five birdies to join Spieth atop the leaderboard in Fort Worth, Texas.

The Spanish star and 2001 Charles Schwab Challenge winner has opened the event with a 63 or better on two previous occasions, both of which came at the AT&T Byron Nelson in 1999 and 2016.

Erik Compton and Jason Kokrak are the duo's nearest rivals, while Patrick Reed and Justin Rose are two under through 18 holes – a shot better off than defending champion Collin Morikawa, while Justin Thomas posted a first-round 72.

As for six-time major winner Mickelson, the American star had to settle for a three-over-par 73.

Mickelson became the oldest major winner in golf history thanks to last week's two-stroke victory at the PGA Championship in South Carolina.

The 50-year-old, who eclipsed Julius Boros (48 years and four months at the 1968 PGA Championship), had not won on the PGA Tour since 2019, while his last major triumph came via the 2013 Open Championship.

After defying age and form, Mickelson bogeyed five holes to start his Charles Schwab Challenge campaign but he brushed off the display.

Mickelson said: "Yeah, I didn't play well. I shot three-over. But I won the PGA, so ..."

"You can't play this course out of the rough because then you have tree trouble, which I had repeatedly, and I didn't putt well," added Mickelson. "But I won the PGA, so I'll see if I can get it turned around for tomorrow."