“It’s a task. It always has been my entire year playing in hot conditions trying to keep my weight.
“So, that’s why I went to the gym yesterday, did some treatment and watched TV.”
McIlroy had been a shot clear of Woods at nine-under par after 15 holes when Woods headed for the clubhouse.
But bogeys on the seventh and eighth holes cost the World No8 posting a sub-60 first round.
“What was worst was probably just not giving myself a better chance to shoot the 59,” he said.
“I can accept bogies. Definitely I can accept a bogey on a hole like on the 8th hole and the bogey on seventh wasn’t good but you don’t get many opportunities to break 60 and today was one of them and I didn’t capitalize.”
American Xander Schauffele boosted hopes of securing next Monday’s very last spot into the USA Ryder Cup team to be in third place with a 63 while Rickie Fowler, who played alongside Woods, returned from injury to shoot a 65.