Hall, ranked 197th in the world, carded eight birdies and 10 pars on a flawless bogey-free round to open up a commanding four-shot lead in the clubhouse at Colonial Country Club in Fort Worth.

Hall, who is from Cornwall in southwest England, rattled off five birdies on the front nine to reach the turn at five under before pocketing three more down the stretch -- including a birdie on 18 -- to create clear daylight at the top of the leaderboard.

"I was really in the moment out there and determined to play some good golf," said Hall, whose round included a 30-foot putt to save par at the 15th and a brilliant chip-in on the 12th for a birdie.

Hall admitted he was as surprised as anyone at his stellar round.

"I don't tend to see anything coming," he quipped. "But I played some good golf this season, and I've had a couple of top 10s.

"This feels nice to be on the leaderboard. Hopefully I can keep the gap there between the rest of the field this afternoon."

Hall's nearest rival in the clubhouse is Tom Hoge, who is four shots back after shooting a four-under-par 66.

Nine other players, including world number one Scottie Scheffler, were a further shot back on three under after shooting 67s.

Scheffler, who returned to the top of the world rankings after his share of second at the PGA Championship on Sunday, was slow to get going, making just one birdie on the front nine.

But three birdies after the turn, which included making a 30-foot putt on his penultimate hole, elevated the world number one's round to leave him five shots behind Hall.

"I don't know how to describe my round today," Scheffler said.

"I felt like I could have shot really low, and I feel like I could have went in the opposite direction too really fast.

"Anything in the red around Colonial is typically a pretty good score."