Billy Horschel had six birdies in a four-under par 68 on Friday to seize a two-stroke halfway lead in the US PGA Tour's $6.2 million Texas Open.
Horschel's eight-under par total of 136 gave him a two-shot lead over Daniel Summerhays, Charley Hoffman and Australia's Steven Bowditch.
It was a further stroke back to a group of seven players sharing fifth place on 139, which included Northern Irish star Rory McIlroy.
McIlroy, toppled from the world No. 1 spot by Tiger Woods in March, added the Texas Open to his schedule to give himself more rounds before next week's Masters, the first men's major of the year.
He was feeling good about that move on Friday, after a five-under par 67 that included seven birdies and two bogeys.
"I definitely feel like I made the right decision to come here, and even if I hadn't been in this position and have been a few shots back I'd still feel the same way," McIlroy said. "I haven't really been in contention this year. So it will be nice to get into the mix."
McIlroy birdied his last three holes, draining a 26-foot putt at the 16th and a 12-footer at 17.
He reached the green of the TPC San Antonio's par-five 18th in two and just missed a 25-foot eagle attempt.
Two-time major champion McIlroy is among the few golfers to reach 18 in two this week, and he was also on the green in two at the 592-yard, par-five eighth -- but missed his 12-foot eagle attempt.
"I think that shows where my game is," McIlroy said. "Those two par fives are probably the toughest to hit in two with how small the greens are. It shows that my ball-striking is there."
Horschel birdied his last two holes for his second straight 68, dropping birdie putts of more than 10 feet at both the 17th and 18th.
In search of a first PGA Tour victory, he's hoping to keep building on the momentum of a runner-up finish in Houston last Sunday.
Hoffman climbed up the leaderboard with a 67 that included six birdies and a bogey. Bowditch carded a 69 as did Summerhays to share second on 138.
Among the players joining McIlroy on 139 were South Korean K.J. Choi, South African Retief Goosen and Zimbabwe's Brendon de Jonge.