The 26-year-old had been struck down by food poisoning, which McIlroy has attributed to a club sandwich he had eaten in a hotel.
"Who would have thought the eel would be the safer bet!? Hopefully I'll feel better tomorrow," the Northern Irishman tweeted along with a photo of part of a menu.
The four-time major winner was a doubt to take part in this weekend's competition -- for which the winner receives $1.4 million -- after withdrawing from Wednesday's pro-am.
Stomach pain restricted McIlroy to just a few practice shots but the world No. 3 came out swinging when the competition began on Thursday, despite losing 10lbs in just 48 due to his illness.
"I stepped on the scales, I shouldn't have done it but I'm under 70 kilos now for the first time in a long time," he said.
"(I weigh) 150lbs, 68 kilos. I can't remember the last time I was this light. I think it is more a case of rehydration now. Or a few burgers maybe, I don't know."
McIlroy admitted even he was surprised by his impressive first round score, after revealing he had been bedridden for the best part of two days.
"It was probably a little better than I was expecting out there to be honest," he said. "When I was hitting balls this morning on the range I had not been out of bed for basically 48 hours, so I was really stiff."
"I needed to loosen up quite a lot, but when I got out there I felt pretty good."
McIlroy has already had to battle back to fitness once in 2015 after damaging ankle ligaments while playing football, an injury that prevented him from defending his Open Championship title in July.
Prior to tee-off, the Northern Irishman was in bullish mood despite his illness, thanks in part to his impressive record at the WGC-HSBC Champions event combined with his doctor's prescription.
However, McIlroy was left lamenting missed chances in the opening half of his round.
"I'm disappointed I missed three good chances coming in there on the front nine, so it was a score I thought could have been a lot better," he explained.
"But considering the position I was in this time yesterday, it's not a bad start."
South African Branden Grace leads after round one, thanks to a flawless nine-under-par 63.
"It was a fun day," the world number 22 said. "From the start I started hitting the ball nicely and missed a couple of short putts which were costly.
"My score could have been lower but I made a couple of nice ones that canceled the others out. The ball-striking was probably the best it's been in a long time, so that was the main key today."