Miami - World number five Rickie Fowler fired another bogey-free 66 on Friday to take a one-shot lead after two rounds of the US PGA Tour's Honda Classic.
Fowler's second straight four-under effort on the Champion course at PGA National in Palm Beach Gardens, Florida, gave him an eight-under total of 132.
American golfer Jason Bohn suffered a mild heart attack during Friday's round after complaining of chest pains on the course.
The 42-year-old was transported by ambulance to a Palm Beach Gardens hospital for tests and was to remain overnight in stable condition.
Fowler was one stroke in front of fellow American Jimmy Walker, who capped his 66 with a birdie at 17 and an eagle at the par-five 18th.
"I've been swinging well," said Fowler, whose strong early-season form included a victory in Abu Dhabi and a playoff loss to Hideki Matsuyama in Phoenix.
"It's nice to hit fairways, hit greens, especially at a place like this where it's very demanding tee to green," added Fowler.
"I felt like I didn't hit it as good as yesterday, but I hit some really good drives through the middle, through the end of the round, and made a few more putts, which was nice," he said.
First-round co-leader Sergio Garcia, playing alongside Fowler, signed for a 69 for 134, and Australian Adam Scott powered up the leaderboard with a bogey-free 65 that included an eagle and three birdies and put him in fourth on 135.
"I felt like I played quite well. Obviously the course is not getting any easier as the week goes on. The wind was not as much as yesterday but it was still challenging," said Garcia.
The Jack Nicklaus-designed course got the better of Rory McIlroy, whose second straight two-over 72 included a triple-bogey six at the par-three fifth and saw him miss the cut with a four-over total of 144.
It was another disappointing finish in a tournament where Northern Ireland's McIlroy has experienced wildly fluctuating fortunes.
McIlroy's 2012 victory at PGA National propelled him to number one in the world for the first time.
Defending his title the following year, he was struggling when he withdrew complaining of a painful wisdom tooth. After missing out in a playoff in 2014, he missed the cut last year.
In fact, Fowler was the only one of four top-10 players to make the cut. Along with world number three McIlroy, ninth-ranked Patrick Reed and 10th-ranked Branden Grace will also miss the weekend.
Playing alongside McIlroy, defending champion Padraig Harrington of Ireland rebounded from his opening 73 with a gritty two-under-par 68 that had him heading into the weekend on one-over 141.
Harrington nabbed seven birdies to offset three bogeys and a double bogey.
Fowler became the first player to go bogey-free over the first two rounds of the tournament since it moved to the Champion course in 2007.
He rolled in a 22-foot birdie putt at his final hole and Walker, playing behind him, heard the roar but stayed in touch with his strong finish.
"I think it's important to be close to the lead in a tournament like this, because it's such a hard golf course," Walker said.