Scott, who started the third round with a seven-shot lead, missed three par putts inside 8 feet and had to settle for a 1-under 71, which narrowed his lead to three shots over Keegan Bradley going into the final round at the Arnold Palmer Invitational.
The greens were firmer than ever after three days of sunshine, though the pins were accessible and allowed for good scoring. Several players took advantage. Scott did not. His score was helped by a pair of long birdie putts on the back nine.
Bradley's approach to the 18th narrowly cleared the rocks that frame the lake, leaving him a 4-foot birdie putt for a 6-under 66. He will be paired with Scott in the final round, with much at stake for both of them.
A victory for Scott should push him to No. 1 in the world when he arrives at Augusta National to defend his title in the Masters. Bradley hasn't won anywhere in the world since the Bridgestone Invitational in 2012.
Scott was at 15-under 201.
Bradley isn't the only player who now has a chance.
Matt Every (66) and Jason Kokrak (67) were four shots behind, both going for their first PGA Tour victory. Chesson Hadley and Francesco Molinari of Italy each had a 69 and were another shot behind. Hadley, who won the Puerto Rico Open two weeks ago, can qualify for the Masters with a high finish. He likely would need to be in sixth place or better to be solidly inside the top 50 in the world.
Scott was never satisfied with the seven-shot lead, and he still felt comfortable with a three-shot advantage going to Sunday.
"When you've got the lead, you have to work for it,'' he said. ''I'm still in good shape.''
Five holes into the third round, his seven-shot lead already had been trimmed to one. Scott three-putted from 60 feet on the opening hole, an indication of how fast the greens have become at Bay Hill, and he hit a poor chip to 12 feet on the fifth hole to drop another shot. Hadley applied the early pressure with four birdies through the sixth hole to get within one shot.
Scott laid up on the par-5 sixth and hit wedge to 2 feet. And after a few long birdie putts on the 10th and 15th holes restored the cushion, he two-putted from 60 feet for birdie on the par-5 16th to bring his lead back to five shots.
Bradley birdied the 18th. Scott missed a 5-foot par putt on the 17th, and then the 33-year-old Australian nearly had one more wobble. His birdie putt on the 18th slid about 7 feet by the hole. Scott made that coming back for par, which he hopes will be a small measure of momentum he can carry into the final round.
''I missed two par putts shorter than that,'' he said. ''To miss another would have opened the door a little bit too much for my liking.''
As for his health?
Scott, who began the week battling flu-like symptoms, said all he has left is a sore throat.
''And a sore throat doesn't hit the golf ball,'' he said.
Bradley made six birdies on the back nine, including the last three holes. He had a good look at eagle on the 16th and missed the 15-foot putt, made a putt just inside 25 feet for birdie on the 17th and then took on the flag on the 18th, even though caddie Steve ''Pepsi'' Hale was wanting him to play slightly more conservatively.
''I was going right at it,'' Bradley said of his 9-iron. ''He was nervous. I knew the whole way it was going to cover and be perfect. But it worked out.''