Donaldson, who overcame wind and rain to win his first European Tour title in last month's Irish Open at Royal Portrush, ground out a one-over-par 73 in strong gusts at Kiawah Island Golf Resort to edge up the leaderboard.
The average score among the early starters was just under 78 and the Welshman had climbed into the top 20 - and likely to rise higher by the end of the day - after posting a two-under total of 142.
"It was really difficult," Donaldson told reporters after carding four birdies, three bogeys and a double-bogey at the par-five seventh. "We had a lot of wind at the start but it just got a little bit more windy towards the end.
"You think you have a chance towards the end where it's blowing downwind, but the wind was so strong, you just couldn't get near flags or greens.
"It was a result if you actually hit the green on the downwind holes. It's just a brutal test of golf. With rain coming in, you don't get a much harder test of golf than this golf course."
Asked what had been the toughest challenge on the ultra-long Ocean Course as winds gusted up to 30 miles per hour, Donaldson replied: "Staying patient.
"It's just so hard reading the green, never mind getting near the flag when you have to get up and down from there. It's very, very difficult, and frustrating at times.
"You've just got to stay patient, keep hitting good shots and putting yourself in position."
Donaldson, who missed the cut at Atlanta Athletic Club last year in his first PGA Championship, said he liked his position going into the weekend at Kiawah Island.
"We have two more days to grind," the 36-year-old added. "Just try to take it one shot at a time, stay in the short stuff and see if we can make a few birdies."