With thick rough on a long course, par was at a premium.
Jim Furyk, Nicolas Colsaerts and Ben Kohles were at 69.
No one else managed to break 70.
Horschel tied for fourth in the U.S. Open two weeks ago at Merion.
Lucas Glover had a 72 and said it was a very quiet morning — not because Tiger Woods wasn't around to attract a big crowd, but because not many players were making a lot of birdies.