Rory McIlroy and Justin Rose are two shots off the early pace at the Honda Classic, after Ian Poulter pulled out with pneumonia.
Rose and McIlroy, who will rise to world number one if he wins, both shot four-under-par rounds of 66.
American veteran Davis Love leads the tournament in Florida after a 64.
Poulter, due to play with Tiger Woods and Lee Westwood, tweeted on Monday: "Just been for a chest X-ray - got pneumonia guys. Rest time for me."
McIlroy missed the chance to rise to the top of the world rankings for the first time when he lost to Hunter Mahan in the WGC Match Play final on Sunday.
But, with current number one Luke Donald resting this week, the Northern Irishman can move above his English rival if he wins at Palm Beach Gardens.
And he made a terrific start to his bid with three birdies before the turn in his opening round.
Starting on the back nine, McIlroy grabbed his first birdie after a 165-yard approach to five feet on the 14th.
Then he sunk a 12-foot putt on the short 17th before two-putting the 556-yard 18th for an opening 32.
Further birdies came on the second and seventh, with his only dropped shot coming on the eighth when he three-putted from more than 50 feet.
Rose joined McIlroy after hitting back from an early double bogey with an eagle and four birdies.
A wild drive down the second ended in a six for the Englishman, but he immediately made amends with an eagle on the next from nine feet.
It triggered a hat-trick of birdies from the fifth to put him firmly into contention before picking up another at the 16th.
Poulter told his Twitter followers that he was feeling feverish just hours before the tournament started on Thursday and admitted that he might not be able to play.
After pulling out he added: "Sorry guys never been so disappointed pulling out of a tournament. Never felt so rough."
Poulter's place went to Argentinian rookie Miguel Angel Carballo.
Ryder Cup captain Love, who insists that he will play against Europe in September if he qualifies, enjoyed a superb day which included a hole-in-one on the 217-yard fifth.
Three successive birdies from the seventh helped him turn in 31 and another on the 17th cancelled out his bogey on the 10th.
