Kaymer led by one again when Björn bogeyed the 12th and the World Number Five missed a golden chance to double his advantage, three-putting the long 14th for par. His second putt was only two feet.
That gave Donaldson extra hope as he had birdied the 11th and 12th to join Björn one behind, but McIlroy was three back after bogeying the tenth.
Kaymer also failed to birdie the long 15th, missing from five feet after going from rough to sand, and he was joined when Björn and Donaldson made their fours on the 595-yard 14th.
They came in different ways, though. While Donaldson two-putted, Björn was fortunate not to go in the pond on the right and holed from ten feet.
If that was lucky then his playing of the 15th certainly was not. He fired in a fairway wood over the guarding bunker to ten feet and made the eagle putt to lead by two at 18 under.
Suddenly he was on course to be the first back-to-back winner of The European Tour season.