In the other game Ian Poulter, conqueror of current number one Lee Westwood, trailed Nicolas Colsaerts by one after the Belgian's two at the short second.
Donald's opening birdie was matched by Kaymer, whom he beat 3 and 2 in the final of the WGC-Accenture Match Play in Arizona in February, but the German bogeyed the next and pars on the following two were not good enough.
Victory over the other is not enough for Donald or Kaymer to take over at the top. They need to win the final as well for that.
Kaymer had spoken of Donald never making mistakes, but he did on the long fifth and by running up a double bogey seven the lead was back to two.
Colsaerts, who had chipped in after badly missing the second green, remained one up on Poulter after both birdied the driveable fourth.
A ten foot birdie putt that only just made it to the hole won Donald the eighth and another on the ninth kept him three ahead as Kaymer also birdied from five feet.
Poulter was in more trouble when he bogeyed the fifth and then saw his 28 year old opponent - Europe's longest hitter last year - hit an approach to three feet on the seventh to go three up.
Donald went four up when Kaymer went into sand at the short tenth and then rolled right off the green with his second shot.
He double-bogeyed there, but the next was halved in birdies and the 224 yard 12th in pars, with Kaymer holing from 14 feet and Donald from nine.
The German pulled his drive into the hazard off the next and did wonderfully well to salvage a par after taking a penalty drop, but Donald halved it with a six footer and was four up with only five to go.
Colsaerts was much the longer at the 574 yard eighth, but went flying through the green with his second and his first chip came back down the bank into a worse spot.
Poulter won it with a par five and, by halving the next three, stood two down with seven to play.
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