Rory McIlroy is tied for second in the Honda Classic in Florida after carding a four-under-par first round of 66.
Needing to win to topple the resting Luke Donald as world number one, McIlroy set the early clubhouse lead, although out on the course Americans Davis Love and Dicky Pride were both five under.
Justin Rose joined the Irishman after hitting back from an early double bogey with an eagle and four birdies.
Ian Poulter was due to play with Tiger Woods and Lee Westwood.
Last night he had tweeted: "Heading straight to bed, 101.2 temperature, freezing then sweating. I hope I can sweat it out overnight or it could be home for me." He even took a picture of the thermometer.
His place went to Argentina's Miguel Angel Carballo, a PGA Tour rookie ranked 299th in the world.
McIlroy grabbed his first birdie after a 165-yard approach to five feet on the 14th - his fifth - and added a 12-foot putt on the short 17th before two-putting the 556-yard 18th to turn in 32.
Further birdies came on the second and seventh, but he was more than 50 feet from the flag on the next green and three-putted to drop his only stroke.
Rose's double bogey came after a wild drive down the second, but he eagled the next from nine feet, then had a hat-trick of birdies from the fifth and picked up another second at the 16th.
Woods and Westwood were among the later starters and Woods kicked off with a birdie putt of nearly 20 feet.
Ryder Cup captain Love, who insists he will play against Europe in Chicago in September if he qualifies, holed-in-one with a five-iron to the 217-yard fifth and with three successive birdies from the seventh turned in 31.
The 47-year-old bogeyed the 10th, but birdied the 17th to be alongside Pride, who still had two holes to play.
Brian Davis was in with a 68. He reached three under, but had to take a penalty drop on the eighth and bogeyed the hole like McIlroy.
On announcing his withdrawal on his Twitter site and before going for an X-ray Poulter tweeted: "Sorry guys - never been so disappointed in pulling out of a tournament. Never felt so ruff."
The Ryder Cup star had hoped that being paired with two former world number ones this week would put him back in the limelight after a disappointing start to the season.
He took time out to await the birth of his fourth child last month, then managed only 45th and 50th in his first two events back before being beaten by Korean Bae Sang-moon in the first round at last week's Accenture Match Play Championship, the event he won two years ago.
Poulter found himself under scrutiny there from the likes of television commentators Nick Faldo and Bruce Critchley and tweeted: "I'm honoured that people are talking that I'm not working hard enough on my game.
"I have to answer that question, though. I have never worked harder than I am right now - not only on my game, but on my health and fitness.
"There are areas of my game which we have looked at and are making big improvements on. It's just a case of them coming together.
"This game has a funny way of showing hard work sometimes, but it's showing. Trust me, I will deliver like I promised in the Ryder Cup and like I always do."
Next week sees the second of the year's world championships in Miami, but whether he is there remains to be seen.
Love equalled the course record of 64 after chipping to two feet for a closing birdie and led by two from McIlroy, Rose, Pride and two more Americans, Ryan Palmer and Jimmy Walker.
Westwood birdied the second to join Woods on one under, but Padraig Harrington, who had to win just to have a chance of playing in next week's world championship in Miami, bogeyed the first and Graeme McDowell started with back-to-back bogeys.