One day after setting a USGA scoring record with an 11-under 60 at the par-71 Chaska Town Course, Horschel struggled at the par-72 Hazeltine National Golf Club on Tuesday and shot a 6-over 78 in the second round of stroke play at the U.S. Amateur.
Despite giving back an average of one shot per hole compared to his first round, Horschel claimed stroke play medalist honors at 5-under-par 138.
'I'm happy I started off with that 60,' admitted Horschel, whose score on Monday eclipsed the previous USGA record set by Christina Kim at the 2001 U.S. Girls Junior and matched by Loren Roberts at this year's U.S. Senior Open.
'I didn't know what to expect after that round. It was a struggle.'
Mark Harrell carded a 2-over 74 at Hazeltine and finished runner-up in stroke play at 3-under 140. Harrell had a 5-under 66 on Monday at Chaska Town -- the easier course by a consensus.
Hazeltine, a two-time U.S. Open host, is just playing that much more difficult.
'This course is humbling for everyone, even Billy today,' said Matt Ma, who played in Horschel's group and missed the cut for match play. 'But he didn't give up. His misses are always in the right spot. It wouldn't surprise me to see him on top on Sunday.'
Horschel's second round was a far cry from his bogey-free effort on Monday, which included nine birdies and an eagle. The Hazeltine layout produced a double and triple-bogey from the leader on Tuesday.
'I tried a couple of great escapes that didn't work out,' said Horschel, who is using his father, Bill, as caddie. 'I made all of my mistakes today, I hope.'
Canadian Greg Machtaler took advantage of the Chaska Town layout on Tuesday, firing a 4-under 67 to share third place with Kyle Davis at 2-under 141. Davis played Hazeltine for his second round and had a 73.
The field was trimmed to the low 65 players by a plus-four cut line. The match-play portion begins Wednesday at Hazeltine and continues until the winner is crowned following the 36-hole final on Sunday.