Rory McIlroy admitted he was suffering with a stomach bug as he failed to build a commanding lead at the halfway stage of the Dubai Desert Classic.
The 24-year-old upstaged playing partner Tiger Woods with a flawless nine-under 63 that earned him a two-shot advantage after the opening day.
But after being laid low with an upset stomach overnight, McIlroy grafted his way to a workmanlike 70 that left him just one ahead of American Brooks Koepka heading into the weekend.
McIlroy, who lifted his maiden European Tour title at The Emirates in 2009, told the Golf Channel: "It wasn't too bad, there were a few stomach cramps.
"We got some takeaway food last night (Thursday) that obviously did not agree with me and this morning I was not feeling too good before coming to the course, but I don't think it affected the way I played.
"It was a little uncomfortable but it did not stop me from hitting some good shots out there and I am definitely not going to make any excuse for not playing my best today."
Koepka added a seven-birdie 65 to his opening 69 to claim outright second on 10 under, and he is relishing the chance to play with McIlroy for the first time.
"I've never played with Rory, so it's going to be fun," said the 23-year-old. "I know him a little bit from seeing him around, so hopefully I'll be able to play as well as I did today.
"I putted really well today. Yesterday I didn't putt very well, but today my putter came through for me. I've been striking the ball really well, and I also chipped well today. So everything's going well, I can't complain."
Meanwhile, Danny Willett admitted he was taking a more relaxed approach on the golf course after beginning the week with a "nerve-racking" charity skydive.
The Englishman birdied six holes on the back nine as he matched Koepka's 65 and join Damien McGrane and Julien Quesne on eight under, and he insisted his parachuting adventure had a positive effect on his mindset.
"It was a different way to start the week, but if you are standing on the first tee trying to hit a drive on to the fairway it seems a little less nerve-racking than jumping out of a plane at 13,000ft, so it might have prepared me quite well," the 26-year-old from Sheffield said.
"As soon as you jump out you think 'What are we doing?', but after that you level out and start to feel like you are just floating down and it's unbelievable."
LIVE third round coverage of the Dubai Desert Classic kicks off at 9am on Sky Sports 4