Welshman Donaldson, who turned 37 a week ago, had 24 putts and bettered the Lake Malaren course record by one shot.
"I played great from the word go," Donaldson, who captured his first victory in winning the Irish Open in July, told reporters.
"I just felt I was properly zoned in, just felt really relaxed all day and it was a very relaxed atmosphere, good fun playing with (Darren) Clarke as well."
If he ends the year inside the top 50, Donaldson will receive an invitation to make a first appearance in the Masters at Augusta in April.
"I've only ever played three times in the States and they were all this year: the Wyndham Championship, the Bridgestone Invitational and the US PGA Championship," he said.
"It would be a great Christmas present to know that I will be playing in the Masters next year. It would be quite sensational but I need to stay in the present and just keep crossing each bridge as it comes."
Ryder Cup winning pair Peter Hanson of Sweden and Italy's Francesco Molinari were in joint second place with six-under-par 66s in Thursday's opening round.
McIlroy was tied in fourth place with fellow Northern Irishman Michael Hoey and Europe's Ryder captain Jose Maria Olazabal on five under par.
McIlroy complained of a headache in the later stages of his round and blamed it on the heavy pollution in Shanghai.
"I started to get a bit of a headache on the back nine and that's because of all the factories and stuff around here, so we're not breathing in the cleanest air," he said.
"But a good night's sleep tonight and I should be okay for tomorrow."
Olazabal recorded his best first-day score in four years with a bogey-free round.
"It's great to shoot a low round and while the driver was the weakest link, the rest of the game was pretty sharp," the Spaniard told reporters.
"Also I feel some freedom now the Ryder Cup is behind me. It's been a tough year-and-a-half, a lot of things on my mind, decisions to make, meetings to attend. So I'm really glad that it's over; now it's fun time."
- Sports & Recreation