Donald, England's world number one has to finish in the top two at the Palm Course at the Disney World resort to win the US PGA Tour money list race, and he moved to 12 under par.
Birdies at the first and second improved his placing and although he dropped a shot at the par-four fifth, he responded with birdies in the next two holes to move to within three shots of the lead.
However, Webb Simpson - £229,000 ahead going into this event - was also on 12 under after seven shots of his final round.
If Donald wins the American would have to be runner-up on his own to claim the crown, while if the Englishman is solo second or in a two-way tie, Simpson would miss out if he is lower than eighth or 21st respectively.
The American started badly, bogeying the first, but birdies at the fourth, seventh and eighth maintained his hopes of topping the list.
The event was led by Kevin Chappell of the United States, who was 15 under after four holes of his final round, with compatriot Justin Leonard a shot behind and Billy Mayfair a further shot adrift after the same number of holes.