Donaldson closed with three consecutive birdies and is at 13-under 131 after 36 holes.
Seven players are right behind at 12-under, however. That group includes Keith Horne, who had to finish his final four first-round holes Friday and then shot a second-round 3-under 69 for get within one stroke of Donaldson.
Another five golfers are two strokes off the lead.
Horne, whose play Thursday was suspended because of darkness, had a pair of birdies in completing his first round and was tied with Han Chang-won for the lead. Han turned in a 2-under 70 in the second round and slipped to a tie for 14th at 10-under for the tournament. He is one of 19 players within three strokes of Donaldson's lead.
Donaldson opened with a birdie Friday and, after a pair of pars, had birdies on four of the next five holes with a ninth-hole par allowing him to make the turn at 31. He ran off three straight birdies to open the second nine and posted birdies on 16, 17 and 18 for his 61.
Swim Week Miami Beach 2014 [PHOTOS]