England's Justin Rose fired a two-under-par 70 to keep a share of the lead after two rounds of the Arnold Palmer Invitational as Tiger Woods stumbled.
Rose and American Bill Haas stood on nine-under 135 after 36 holes at the US$8.5 million US PGA event, which for Woods and many others is likely the final tune-up event for next month's Masters, the year's first major.
Woods moved within a stroke of the lead only to finish with three bogeys, a disappointing 70 leaving him in a share of seventh, four strokes behind overnight leader Rose and Haas, who fired a bogey-free 66 to leap up the leaderboard at Bay Hill.
"I played good," Woods said. "I missed a couple short ones. The score doesn't indicate how well I played.
"I'm four back. I'm right there. The good news is we've got 36 holes to go. There's a lot of golf left. Certainly four shots can be made up."
Rose birdied the par-three second and par-five sixth, 12th and 16th holes while taking a bogey at the par-three 14th and missing a five-foot par putt at the 18th to fall back into only a share of the lead with Haas.
Woods, who would be world No 1 with a victory this week, opened with a birdie, eagled the sixth and added birdies at the par-four 11th and 13th holes to pull within a stroke of the lead.
But then everything fell apart for the 14-time major champion. He found a bunker off the 16th tee, then went into the water and missed a 27-foot par attempt to take a six.
At the par-three 17th, Woods found the rough left off the tee, punched out 21 feet from the cup and two-putted for bogey.
At the par-four 18th, Woods found the right rough off the tee and left himself 30 feet for par, coming up just short to drop yet another stroke.
American John Huh, who birdied two of the last three holes, was third on 136 with Americans JJ Henry, Ken Duke and Jimmy Walker sharing fourth on 138.
Woods was on 139 with Fiji's Vijay Singh and American Mark Wilson.
Haas, who is due to become a father in eight weeks, birdied the second and par-four eighth, added back-to-back birdies at the 12th and 13th and eagled the par-five 16th to make his charge.
Haas has been bothered by a sore neck for more than a month but showed no sign of distress during his round.
"It's not bad. I can honestly play," Haas said. "I can make a full turn. Sometimes when I look to the left it kind of tightens in the back of the neck. We should call it a sore neck, not an injury.
"When you're playing poorly, it hurts, and when you're playing well, it doesn't hurt."