Horschel's 60 is the lowest score ever posted in a USGA sanctioned event. The previous USGA scoring mark was 62. Christian Kim first set that mark at the 2001 U.S. Girls Junior and was matched earlier this year by Loren Roberts at the U.S. Senior Open.
Horschel, a sophomore at the University of Florida, carded a bogey-free round that included nine birdies and an eagle on the par-71 Chaska Town Course.
Chaska, along with the par-72 Hazeltine National Golf Club, are the two courses being used for the stroke play portion of this championship. Chaska saw six of the seven lowest scores on the day. Hazeltine will host the match- play rounds starting Wednesday.
Horschel birdied two and three, both from over 25 feet out. Of the birdie putts he made on five, seven, eight and 10, the longest putt he sank during that run was from 6 feet.
He poured in a 15-foot eagle putt on 15, then closed with birdies on 16 and 18 for the new USGA scoring record.
'When that last putt went in, I just ducked my head with a big smile,' stated Horschel, whose lowest non-competitive round was a 61 in Florida. 'It was just my day. When I made eagle on the 15th, I started thinking about getting to 59, but I forgot it was a par 71.'
Mark Harrell and Eddie Olson share second place after carding rounds of 5-under-par 66. Dustin Johnson posted the low round of the day at Hazeltine as he carded a 4-under-par 68. He is alone in fourth place.
Shane Prante, Kyle Davis and Mark Sommerfeld each shot 68 as well, but they did so at Chaska. They share fifth place at minus-3. Jordan Cox, Lloyd Saltman, Beck Troutman and Rickie Fowler are tied for eighth at minus-2.
After Tuesday's round, the top-64 players will advance to the match-play portion of this event with starts Wednesday at Hazeltine. After Wednesday's first round of match play, the second and third rounds will be played on Thursday. The quarterfinals will take place Friday, the semifinals on Saturday and the 36-hole final will be on Sunday.