The 23-year-old German, the 2007 European Tour rookie of the year, totaled 17-under 199.
Anthony Wall of England shared the best round with a 65 and moved into second. Henrik Stenson, the runner-up last year, had a 69. Fourth-place Lee Westwood also shot 65 and trails the leader by eight strokes.
Kaymer opened with birdies on his first two holes and later added two more in a bogey-free round.
'When I came here on Tuesday, I didn't like the greens that much. Of course I love them now,' Kaymer said.