Day is set to rise from No.3 to No.2 in the world rankings after battling through the last day for a wire-to-wire, one-shot win in the Arnold Palmer Invitational in Orlando, Florida.
The Queenslander took a two-shot lead into the final round and fired a two-under-par 70 to finish at 17-under 271 and win from American Kevin Chappell (69).
“It means the world,” said Day, who received an encouraging text from Tiger Woods in the morning.
“I just ground out today. It wasn’t the best day that I had, but just to be able to get it done when I wasn’t quite on.”
It is Day’s first win since the BMW Championship late last year and comes at a great time with the Masters looming on April 7.
He will chase back-to-back major championship titles when he attempts to add the Masters green jacket to the US PGA Championship crown he won last year.
Day trailed Chappell by one shot with two holes left to play but turned his fortunes quickly with a birdie and a clutch par save from the bunker on the final two holes.
He hit his tee shot on the par-3 17th to 12 feet and made it for birdie, and then got up-and-down from just under 100 feet away in a bunker to win by one shot.
Chappell was seeking his first win in his 150th career start.
Troy Merritt and Henrik Stenson both faltered late as they carded 71s and tied for third.
Adam Scott was the next best-placed Australian, in a five-way tie for 12th after a 69 final round which included four birdies on the back nine.
He could have finished much higher, if not for a double-bogey on the final hole where he almost found the water at the back of the green.
Scott was in a five-way tie for third after the first round, however a tough one-over second round all but dropped him out contention as he chased a third straight win.
Marc Leishman (69) finished eight-under, in a three-way tie for 17th.
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