The world number three missed a promotional event on Tuesday and withdrew from Wednesday's pro-am competition after being unable to hit balls on the practice range.
"I’m not feeling great to be honest," McIlroy said. "I had something (to eat) a couple of nights ago and yesterday wasn’t good at all.
"I thought I would be better this morning, but I had a piece of toast and a couple of bits of melon for breakfast and I came here and tried to hit some balls, but even just making a couple of swings my stomach was too sore.
"I just need another day of rest and hopefully I will be better tomorrow and I can play.
"I was up from about 1am to 4am last night. The night before it didn’t start until about 4am, so it hasn’t been great."
McIlroy, who finished tied sixth at the Turkish Airlines Open last week, said he had been given antibiotics and painkillers for the abdominal pain.
The four-times major winner is scheduled to begin his opening round at 1045 AM local time (0245 GMT) on Thursday with American duo Dustin Johnson and Rickie Fowler at the $8.5 million event.
"I was thinking of trying to play nine holes, but that would probably do me more harm than good. It would be nice to see the golf course before tomorrow, but I’ve played it a few times," the 26-year-old said.
"I didn’t play last year but I know the place well and (my caddie) JP has been out there, so it is better that I rest and get my energy up for tomorrow."
McIlroy leads England's Danny Willett, Irishman Shane Lowry and South African Louis Oosthuizen in the European Tour's Race to Dubai standings with only three tournaments remaining in the season.
McIlroy will be keen to play this week in a tournament which has yielded him good results in the past. The Northern Irishman finishing no worse than sixth in his four appearances at the Sheshan Golf Club.
"It has been a really, really good tournament for me," said McIlroy, who won the WGC-Match Play event in May before an ankle injury forced him out of his British Open title defence in July.
"I’ve not won it, but I’ve been close. Hopefully if I can get through the first couple of days and play OK, I’ll be feeling a lot better at the weekend and I can make a run at it."
(Writing by Patrick Johnston in Singapore; editing by Amlan Chakraborty)
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