Australia's Jason Day is four shots adrift of a surprise early leader at the PGA Tour's Rocket Mortgage Classic.
Davis Thompson, in his third PGA Tour event as a professional, has tied a Detroit Golf Club record of nine-under 63 in the opening round.
Thompson, a 22-year-old in the field as a sponsor's exemption, leads by two strokes from Brandon Hagy and Tom Lewis, with Seamus Power and JJ Spaun a further shot adrift.
Day carded five-under with compatriots Cameron Davis (four-under), Aaron Baddeley (three-under), Matt Jones (two-under) and Cameron Percy (one-under) all breaking par.
Thompson missed the cut last week at the Travelers Championship after tying for 35th in his pro debut at the Palmetto Championship earlier in June.
And it's unlikely anyone saw his breakthrough round coming after he was a combined six-over par in six previous starts – four as an amateur – on the PGA Tour.
But he matched the course record shared by Nate Lashley, who won the inaugural event in 2019, and JT Poston.
Defending champion Bryson DeChambeau got off to a shaky start, shooting an even-par round.
And Phil Mickelson carded three-under after entering the event for the first time in its three-year history – and then said it will be his last appearance.
Mickelson said he will not come back due to what he called an "unnecessary attack" in a report by The Detroit News that was published earlier in the week.
The newspaper obtained federal court records from 2007, detailing how a Michigan-based bookie was accused of cheating Mickelson out of $500,000.