Spieth shot a 7-under 63 on a breezy Thursday to be tied for the first-round lead at the Charles Schwab Challenge with Sergio Garcia, who had a 15-foot birdie attempt on the 18th hole that circled the edge of the cup but didn't go in.

Both former Colonial champions had bogey-free rounds.

The 50-year-old Mickelson, only four days after becoming the oldest winner of a golf major, was 10 strokes back after a 73 that ended with the 22-foot birdie while playing with local favourite Spieth and defending Colonial champion Daniel Berger (68).

"Yeah, I didn't play well," Mickelson said. "But I won the PGA, so."

Garcia's first PGA Tour win came at Colonial as a 21-year-old. PHOTO: Tom Pennington/Getty Images.

Garcia matched the closing 63 he had at Colonial when he got the first of his 11 PGA Tour wins 20 years ago at age 21.

"It was quite breezy. It was gusty, so it wasn't easy to pull some of the clubs," Garcia said. "There were some tough holes out there, but I was able to hit really good shots on those holes, and a couple up-and-downs when I needed them."

Spieth had a tap-in for his third birdie in a row at the ninth, and seventh overall – and right after a 71-foot chip-in from the rough at the par-3 eighth.

The 2016 winner and two-time runner-up had his best score in 33 career rounds at Colonial, one of two local events for the Dallas native, after rolling in a 12-foot birdie putt to start the day.

"It's hard to go any lower," Spieth said. "That's what I talked about before the tournament.

"If I could get a couple to go in early in the first round, and the confidence, and the work I've been doing on my stroke the last few days ... I thought that would exude just a little bit of confidence into the rest of the round on greens where I've been very successful on before."

Erik Compton, the two-time heart transplant recipient who got into the field on a sponsor exemption, and Jason Kokrak had 65s.

The 41-year-old Compton played bogey-free after birdies on the first two holes of his second PGA Tour start this year. He missed the cut at the Honda Classic.

Mickelson, the 2000 and 2008 Colonial champ, missed nine of 14 fairways – some with irons off the tee – in a scrambling early-starting round that followed an emotionally draining few days.

After winning Sunday at Kiawah Island in South Carolina, Mickelson flew home that night to California, then allowed himself a couple of days to relish his historic accomplishment.

Cam Davis is the best of the Australians, his even-par 70 keeping him in a tie for 41st, one shot ahead of compatriot Matt Jones.