Grace is more than 1.1million points behind McIlroy on the Race to Dubai, but each event in the Final Series offers 1.3million to the winner and the 27-year-old is in a rich vein of form after compiling a perfect 5-0 record in the Presidents Cup in South Korea last month.

A flawless 63 gave Grace a one-shot lead over Thorbjorn Olesen, Kevin Kisner and Steven Bowditch, with England’s Danny Willett – who is second on the money list – another shot back alongside 2013 winner Dustin Johnson and fellow American Patrick Reed.

McIlroy was taken ill on Tuesday and missed the following day’s pre-tournament pro-am, but could call on the experience of his four previous appearances in the event which have yielded four top-six finishes and a combined score of 52 under par.

“(It was) probably a little better than I was expecting out there to be honest,” the world number three 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.

“I’ve lost 10 pounds since being here. I can’t remember the last time I was this light. But hopefully (if I have) a couple of good meals over the next couple days and rehydrate I’ll be feeling a lot better. I’m looking forward to a good dinner tonight but hopefully it will stay inside my body.

“Thankfully it was a decent start, and now I’m going to try to just get into the tournament. 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, but considering the position I was in this time yesterday, it’s not a bad start.”

Grace has won twice on the European Tour this season and was fourth in the US Open and third in the US PGA Championship, both times carding costly double bogeys at vital times in the final round.

“Coming into these events, knowing that I’m sixth on the Race to Dubai, I know one good week can change everything and give it a last final push,” Grace said.

“There’s a lot still to play for.

“It was a fun day. 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 cancelled the others out. The ball-striking was probably the best it’s been in a long time, so that was the main key today.”

Willett, who is almost 400,000 points behind McIlroy, was disappointed to drop his only shot of the day on the ninth – his final hole – but delighted to still be in contention to end the year as European number one.

”I’ve not dropped out of the top two on the Order of Merit all year,” said Willett, who beat McIlroy in the first round of the Amateur Championship in 2007 and played on the same Walker Cup team a few months later.

”I suppose when you win Nedbank (Golf Challenge, in December 2014), it’s a pretty good leapfrog on everyone at the beginning of the season. It’s been nice to be up there. I think Rory overtook us somewhere in the middle and we’ve been trying to claw our way back ever since.

”I played pretty nicely last week, didn’t lose too much ground and again this week, you have to put yourself somewhere near and see what happens. I don’t think he (McIlroy) is playing next week and if you can be within touching distance going into Dubai, Dubai could be pretty interesting.”

World number two Jordan Spieth, who can reclaim top spot from the absent Jason Day with a top-10 finish, matched McIlroy’s 68 to finish alongside defending champion and playing partner Bubba Watson, who celebrated his 37th birthday with a bogey-free round.