Gordon, a 23-year-old who is in the event due to a sponsor's exemption, turned in the best round of his young career.
"I've hit some good wedges, but more important than that I've given myself fair chances because I've hit a lot of fairways", Mickelson said. "But as a personal milestone it would have been neat". Hughes made 10 birdies en route to a 10-under 60 to sit atop the leaderboard by three shots over Rory McIlroy, Xander Schauffele and Viktor Hovland following Thursday's opening round.
Four other players are grouped with Mickelson on 12 under.
There were 106 players who broke par. Patrick Cantlay was the last to shoot 60 at TPC River Highlands as an amateur in 2011.
Gordon started on the back nine and birdied six of his first eight holes, the first of them on a 45-foot putt at the 10th. He had four bogeys and five birdies.
"It's just been nice to get back into some competitive golf again", McIlroy said. "Who knows when that was going to be". "I felt like I was playing aggressive because of that. Thursdays and Fridays don't feel that different to be honest, but into the weekends they do". "But I've had 30 U.S. Opens". "I think we all need to remind ourselves that we're all learning to live with this virus".
"I don't want to do anything that might jeopardise the health of any player in the field or his ability to compete", said four-times major victor Koepka. His final birdie came on his next-to-last hole at No. 8.
"The greens firmed up a little bit", he said.
Gordon's lone bogey came on the par-4 12th hole.
Nick Watney withdrew just before the second round of last week's RBC Heritage Championship. I'm very happy with the way I played.
Brooks Koepka and Graeme McDowell withdrew from the Travelers Championship on Wednesday after their caddies tested positive for the coronavirus. Koepka and Harmon tested negative, but Elliott was positive.
Monahan delivered his remarks a day after a second PGA Tour player in a five-day span tested positive for Covid-19.
World number four Koepka and 2010 U.S. Open victor McDowell, who competed on the PGA Tour the last two weeks, both tested negative but made a decision to skip the PGA Tour's third tournament back from a COVID-19 hiatus out of caution. The Travelers is the third event on the tour calendar since golf resumed.
Cauley, who opened with a 69, had two negative tests on Friday. "Both times there was an outside chance the last few holes to shoot 59. Playing my game and the way the course set up for me was a challenge when you're playing with guys that hit it like that, but it allowed me to play some good golf".