Now he faces a leaderboard filled with experienced players behind him, including four — McIlroy, Jim Furyk, Retief Goosen and Lee Janzen — who have combined to win seven majors.
"I couldn't care less if they've won majors or not, honestly," Horschel said. "I know they're good players and what they've done, but I feel I'm a good player in my own right.
"I think you have that mindset. You can't think that these guys have won majors, and then you start worrying about what they're doing and you forget about yourself."
McIlroy is the most recent major winner of the pursuers, having won the PGA Championship last August. However, he's struggled for much of this year following an equipment change and recently lost the No. 1 ranking to Tiger Woods.
McIlroy has just one top-10 finish this year, and he decided late last week to add the San Antonio stop to his schedule to add competitive rounds before next week's first major of the year.
It's a move that appeared questionable after an up-and-down even-par 72 on Thursday, but not after Friday's scoring binge.
McIlroy still had fits with his driving, hitting only nine of 14 fairways, but he made lengthy birdie putts on each of his final three holes — including a 26-footer on the par-3 16th. He narrowly missed a 24-foot eagle putt in front of a surging gallery on the 18th after reaching the par 5 in two shots.
It was after his run-in with the cactus on No. 9 that McIlroy made his move, shooting a 4-under 32 on the back nine.
On the ninth, he drove into the left trees and scraped his left shin on the prickly pear while searching for his ball. McIlroy rubbed his leg afterward and his approach went the right of the green on the par 4, leading to his final bogey of the round.
"I shouldn't have been where I was at nine anyway, so I guess it was deserved," McIlroy said.
McIlroy's 5 under matched Hoffman and K.J. Choi for the best round of the day. The Texas weather is expected to warm considerably on Saturday and the wind to pick up — making the 7,435-yard layout even more challenging.
"I just like giving myself a chance coming in on Sunday," Hoffman said. "I think I'm a pretty good competitor. If I have a chance with a few holes to play, that's all I can ask for."