In a match suspended briefly on the fourth because of a hailstorm - there had been snow during the night - Donald won three of the first five holes.
But his Ryder Cup teammate, whose semi-final victory over Bubba Watson meant he dethroned Lee Westwood at the top of the Official World Golf Ranking, took the short sixth with a par, the long eighth with a birdie and then the ninth with a bogey after Donald came up short in the desert scrub and ran up a triple-bogey seven.
Donald had still not been behind all week, though, as he chased a first victory in America for five years.
An 18 footer for birdie at the second had put him in front and he hit his approach to within three feet of the flag on the fourth when play resumed after a ten-minute hold-up.
His German opponent then drove into the wilderness on the next and had to take a penalty drop.
Donald had yet to play the final hole and in his two previous games had seen off Ryan Moore 5 and 4 and then Matt Kuchar 6 and 5.
In all he had played 73 holes to Kaymer's 85, but it was less of a factor with the final being cut from two rounds to one this year.
Victory for Donald would take him all the way from ninth in the world to third behind Kaymer and Westwood, but even if he lost he would move to sixth and equal his career high.
Kaymer was trying to make it five wins in his last 11 starts, a run that began with his first Major title at the US PGA Championship last August.
In the all-American third place play-off, meanwhile, Kuchar led Watson by two with eight to play - after double-bogeys for both of them on the tenth in the cold and windy conditions.
Donald did well to save a half at the tenth after finding more trouble and Kaymer was favourite to take the 11th as well, but missed from six feet after his opponent had holed from nine.
One down again as a result, the 26 year old - second youngest holder of the World Number One position after Tiger Woods - strayed into sand on the 219 yard next and by bogeying fell two down with six to go.