Kaymer, who had led by six shots going into the final round, slipped to a 2-over-par 74 but still won by four at 15-under 273.
The young German was chosen as the tour's top rookie last year.
"It is just my second season on tour and it feels unbelievable that I can win this tournament," Kaymer said. "I felt the pressure early on and I made a couple of bogeys, but from there I was able to stick to my game plan."
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