Oosthuizen, the 2010 British Open champion, was five over par when he was carted off at East Lake Golf Club.
The 32-year-old South African, who also had his wrist drained on Wednesday to treat a cyst, said he had been bothered by the hamstring earlier in the week but felt fine at the beginning of the opening round.
"Starting the round I didn't even feel it, I didn't even know about it," he said. "But after five or six holes, I started feeling it immediately and started walking slower and just every hole it got worse and worse and worse."
He said that by the 13th hole "I started swinging it different because I felt I struggled to go through on the leg and that's when I decided to withdraw".
Oosthuizen said the wrist did not give him any problem on the course, but the South African was concerned about his availability for the Oct. 8-11 Presidents Cup in South Korea.
"It was getting worse and once it gets to a point where you don't swing it proper, it's no point in playing. That's when you start losing confidence and you start hurting other things," he said.
Oosthuizen said the hamstring casts some doubt over his plans to compete on the European Tour in Scotland next week.
"I still would love to play next week, Dunhill Links, that's a big concern now, because that's also a tough walk, those three golf courses," he said, adding he might have to give it a miss to prepare before Presidents Cup.
Oosthuizen said that his physio, who is with him in Atlanta, would accompany him home to Palm Beach and work on the injury over the weekend.
"It's not just the individual thing, the Presidents Cup. It's a team thing, so I need to make a call on next Wednesday or Thursday and tell (International team captain) Nick Price if I'll be a hundred percent or not."
The South African had qualified 29th out of 30 players for the season-ending Tour Championship, which also decides the winner of the FedExCup playoffs and $10 million bonus prize.
(Reporting by Larry Fine; Editing by Frank Pingue/Mark Lamport-Stokes)
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