Scott began the day with a seven-shot advantage. He drove the ball superbly again, but also hit a few loose iron shots and a couple of poor putts on his way to a one-under-par 71 in ideal conditions at Bay Hill in Orlando.
"They don't hand you PGA Tour events when you've got a lead after 36 holes. You've got to work for that," he told Golf Channel after posting a 15-under 201 total with one round left.
American Keegan Bradley finished with birdies at the final three holes to shoot a 66 and jump into second place on 12 under. Four other players are within five strokes and realistically in the hunt.
Scott will defend his Masters title April 10-13 as the new world number one if he wins on Sunday, and he remained upbeat about his chances after sinking a seven-foot putt to save par at the par-four 18th, a hole he bogeyed the first two days.
"That's huge going into tomorrow," he said. "I missed two putts of shorter length for par on the back nine today and to miss another one would have opened the door a little bit too much for my liking.
"I got off to a tough start (with two bogeys in the first five holes) and just fought my way back all day. No doubt everyone made a move today and I was battling but I'm in good shape for tomorrow."
Scott, who has been under the weather all week, said he was starting to feel a little better physically.
"I'm feeling OK to play. I've got a really sore throat but that doesn't hit the golf ball."
Bradley, the 2011 PGA champion, had no illusions that it would be easy to catch Scott on Sunday.
"I'm going to have to shoot a really low score, because Adam's one of the best players in the world, but that's what I live for," he said after matching the day's best score.
"I've been hitting the ball nicely all week but the last two days the putter has gotten really hot, which is a good sign."
American Chesson Hadley got closest to Scott, charging within one stroke after he birdied the sixth hole, but he bogeyed the next and ended the day five strokes behind after a 69.
Italian Francesco Molinari is also five back after almost making what would have been a miraculous eagle at the par-four 15th, where his 180-yard approach shot from the thick rough clattered against the pin but did not drop, leaving him to settle for a tap-in birdie.
- Sports & Recreation