Donald shot a 5-under-par 67 Friday and is at 12-under 132 after 36 holes. Rhys Davies, who shot 68 in the second round, is one stroke back overall and Jamie Donaldson (70) is in third at 9-under 135.
Davies and Donaldson were tied with Donald for the first-round lead.
Donald, who is leading a European Tour event at the halfway mark for the second consecutive week, built his lead with strong approach shots. Playing the back nine first Friday, he birdied five of the six holes from No. 11 to No. 16 and didn't have a birdie putt longer than 4 feet in the stretch.
He moved to 7-under on the day with birdies at Nos. 2 and 6 but posted bogeys at the seventh and eight holes.
"It's hard to play perfect golf for four days but the two bogeys were not really bad shots and I feel like I have a lot better control with my irons this week," Donald said.
Davies missed a chance to tie for the lead when he had a bogey at 17 and Donaldson overcame early problems with three straight late birdies to stay in contention.