Donald looked set for a second consecutive 68, or better, after playing his first 15 holes in four under par with five birdies coming after a bogey on the second.
But the world number one then dropped a shot on the 16th after missing the green with his tee shot and fluffing a chip, while another shot went when he surprisingly three-putted the par-five 18th from short range.
A round of 70 left Donald on six under par for the tournament, five shots behind England's Lee Slattery, who added a 66 to his opening 67.
Slattery lost his card by just £66 in 2007 and at 136th on The Race to Dubai faces another nervy end to the season.
He carded seven birdies and one bogey, bouncing back from his only dropped shot at the 12th with a monster birdie putt on the 14th.
"I got off to a good start, I was two under early and just kept it together and had a nice finish, birdied the last two holes," Slattery told reporters at the course.
"It's mainly down to putting. I putted very well today, seemed to hole everything. I drove the ball well. On this course you can give it a good smack off the tee and you feel yourself trying to hit it harder and harder.
"I was actually getting too cautious with my game but round here you can't be, you just whack it. This suits me because I just enjoy it and don't think too much - that's the key. I don't think I've ever walked off the course feeling so relaxed."
Slattery enjoyed a one-shot lead over Italy's Francesco Molinari and Spain's Eduardo de la Riva, who both carded second consecutive rounds of 67.
Molinari, a member of the victorious Ryder Cup team last year, said: "It was a good solid round, I'm hitting the ball better than I was this summer.
"Today's round was trickier than yesterday's because of the wind. I'm even more pleased today. Five under is a good score in any conditions."