Bill Haas fired a 6-under 66 on Friday to move into a share of first place with Justin Rose at the Arnold Palmer Invitational.
Haas, who is battling neck and shoulder injuries, started the day four strokes behind the first-round leader Rose, but steadily climbed the leaderboard with his bogey-free round to grab a share of the lead at 9-under-par 135.
"We should call it a sore neck, not an injury," said Haas. "When you're playing poorly, it hurts, and when you're playing well, it doesn't hurt."
Rose carded a 2-under 70 during his second round at Bay Hill Club & Lodge.
John Huh (69) sat alone in third place at 8-under, while Ken Duke (68), J.J. Henry (67), and Jimmy Walker (69) shared fourth at minus-6.
Tiger Woods (70) was within a stroke of the lead following his second birdie in three holes at the 13th, but he would get no closer and closed out his day with three straight bogeys to fall into a tie with Vijay Singh (68) and Mark Wilson (68) for seventh place at 5-under.
Starting on the back nine with one of the early groups, Haas started to make his move by capping consecutive birdies at the 12th and 13th with a 31-foot putt.
He then hit the turn at 4-under on the day after hitting his second shot at the par-5 16th within nine feet and converting the eagle attempt.
A 9-foot birdie putt at the second moved him further up the leaderboard before he followed five straight pars with a 21-foot birdie putt at the eighth to move into the top spot.
He closed his round with a par at the last to head into the clubhouse with the lead.
"Yesterday was one of my better ball-striking rounds of the year and I just finished it off poorly," said Haas, who bogeyed three of his last five holes during his opening round. "Today it was kind of my goal to get those back and once I did, from then on, I was just trying to hit good golf shots."
Rose, however, chipped in from seven yards out at the second and tapped in a short birdie putt at the sixth to move into a share of first place.
He briefly took sole possession of the lead with a 4-foot birdie putt at the 12th, but 2-putted for bogey from within five feet at the 14th to fall back into a tie with Haas.
Rose then regained the lead at the 16th with a tap-in birdie after his chip from just outside the green settled about six inches from the hole.
But he would again lose the top spot with another 2-putt bogey at the last to head into the weekend tied with Haas.
"I kind of lost my focus. Well, not lost my focus, but just kind of second guessed that putt down the hill a little bit on the speed once the rains fell," Rose said about his final bogey. "But that was the only thing that hampered the day, really. All in all, exciting day, and I'm in a good position."
NOTES: Haas suffered his injuries while reaching down to pick up a towel at the Northern Trust Open ... Haas has had sole possession or a share of the 36- hole lead 10 times. He has only gone on to win once, at the 2010 Viking Classic ... Rose has converted a 36-hole lead or co-lead into a win twice out of 10 opportunities, at the 2010 AT&T Championship and the 2011 BMW Championship ... The cut line was set at 3-over-par 147, with 78 players advancing to the weekend ... Kevin Chappell recorded a hole-in-one at the 14th hole, but he failed to make the cut ... Woods can move to No. 1 in the world rankings for the first time since October 2010 with a victory.