Jason Day could be forced to pull out of this week's Farmers Insurance Open after it emerged he is battling a virus.
Day, who is the defending champion at Torrey Pines, was reported to be suffering from flu-like symptoms as he began his preparations for the second event of the PGA Tour's West Coast Swing.
The world No 2 withdrew from Wednesday's pre-tournament pro-am, but he remains hopeful of being healthy enough to tee off alongside JB Holmes and Harris English on the North Course on Thursday morning.
Day collected the first of his five victories last year at the iconic California venue following a play-off against Holmes, English and 2014 champion Scott Stallings.
English and Stallings were eliminated at the first extra hole, and the Australian clinched victory on the par-three 16th when he laced a superb tee shot to 15 feet and Holmes flew the green and was unable to get up and down for par.
It was Day's first win since his thrilling duel with Victor Dubuisson at the WGC-Accenture Match Play almost a year earlier, and provided the springboard for the most prolific season of his career.
Day went on to win the RBC Canadian Open before landing his maiden major title at the PGA Championship, when he held off Masters and US Open champion Jordan Spieth on the final day at Whistling Straits.
He continued his sublime run of form with a dominant six-shot win at The Barclays, where he was also forced to miss the midweek pro-am after suffering a back injury during practice.
Day made it four wins in less than two months with another six-shot triumph at the Deutsche Bank Championship, although he missed out on the $10m FedExCup jackpot when Spieth won the Tour Championship at Atlanta.
The 27-year-old is the 13/2 favourite with Sky Bet to retain his Farmers Insurance Open title this week, although Abu Dhabi winner Rickie Fowler is among the strong field which also includes Dustin Johnson, Justin Rose, Phil Mickelson, Patrick Reed and an in-form Brandt Snedeker.