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."