Mackenzie Hughes narrowly missed out on a round of 59 and world number one Rory McIlroy recorded an impressive seven-under 63 as the pair set the pace at the Travelers Championship.
Canadian Hughes’ putt at the last fell short after a 10-under par round of 60 that contained 10 birdies.
McIlroy, Northern Ireland’s four-time major winner, carded an eagle, six birdies and a bogey for his 63.
American five-time major winner Phil Mickelson is in contention after a 64.
The event went ahead despite three individuals testing positive for coronavirus.
Cameron Champ, Graeme McDowell’s caddie Ken Comboy and Brooks Koepka’s caddie Ricky Elliott have the virus and withdrew from the tournament in Cromwell, Connecticut.
PGA Tour commissioner Jay Monahan said the news acted as “stark reminder” to comply with safety measures.
McIlroy started on the 10th and was three under for his opening nine holes and one better coming in.
Tyler Duncan and Michael Thompson finished on six under with fellow Americans Bryson DeChambeau, Russell Henley and Tyler McCumber all posting 65s.