Jason Day snatched the Arnold Palmer Invitational at Bay Hill to take over from Rory McIlroy as world No 2 and underline his credentials as a major contender for Masters glory.

As Rory McIlroy closed with a spectacular 65 to prove his game is not far away as Augusta National approaches, Day brilliantly birdied the 17th to take the lead and then got up and down from sand at the last, holing a clutch four footer for a 70 and a one shot win over Kevin Chappell on 17 under par.

“This is a really rewarding win for me,” Day said. “To be able to get in the house and make the par on the last was very satisfying.” 

It was the eighth PGA Tour win of Day’s career, the fifth in his last 12 starts and his third since he captured the US PGA at Whistling Straits last year.

It also ended all doubts about his form.

"It does a lot of confidence for me knowing that everyone was asking what's wrong, what's going on, why aren't you playing well? I just kept on saying to myself, kept on saying to the people, the fans, the media, just be patient, I'm just going through the process and I'm going to keep working hard. Things take time. It happened this week. I'm just happy that I won."

Two ahead of Chappell, Henrik Stenson and Troy Merritt starting the day, he made three birdies and two bogeys in his first six holes before a tap in birdie after a towering approach to the ninth gave him a share of the lead heading into the back nine.

Merritt the birdied the first five holes coming home to get within one of Day, Chappell and Stenson with four to play. But playing alongside Day and convinced he needed a birdie, he ended up finding water and taking six at the last for a 71 to finish fourth on 14 under alongside the Swede, who bogeyed the 14th and 16th for a 71.

Seeking his maiden win, Chappell had birdied the 13th and 16th to lead by one from Day and Merritt on 17 under.

But he drove into the right rough at the 18th and bogeyed for a 69 that left him to settle for solo second on 16 under.

Day was far from his best but he hit a towering, 222-yard tee shot to 12 feet at the par-three 17th and poured in the putt to snatch the lead and then got up and down from the left green side trap at the last to take victory.

As for McIlroy, he rebounded from a third round 75 by firing two eagles and five birdies in a closing 65 to finish tied 27th on six under par.

But admitted that he needed to go easier on himself mentally and improve his course management after racking up six double bogeys over the four days — the worst tally of his PGA Tour career

After signing off by holing 59 footer for his 16th birdie of the week at the 18th, he assessed his rollercoaster start to the season and concluded that it’s not his swing but his decision making and his attitude that’s causing him to make big mistakes.

What was that about anger?

"I think the first day it was more of a shock element more than anything because I was -- I played so well in practice and I practiced well at home and I get here on the first tee on Thursday and hit one OB left. Don't know, where does that come from?

"I just told myself to keep trusting my swing and obviously there's a lot of trouble left and water left on this golf course. I find out a couple times after that. So, yeah, more mentally than anything else. Just need to try to get over it quicker and rebound and not -- just not be so hard on myself or sort of get so angry, I guess."

Asked where his double bogeys were coming from, McIlroy said: “Missing it on the wrong side, not being smart. When I do miss it, not take my medicine, trying to be too cute with it, trying to make up for the bad shot I hit instead of maybe giving myself that 12, 15-foot putt for par.

“And I hit a lot of balls in the water here this week which doesn't help. Just trying to make my bad shots a little bit better.”

Assessing his year — he’s shot under par in 15 of 22 rounds with all six of his over par rounds coming in recent weeks — he said: “It's been the story of the week and sort of been the story of my year as well. I've played some great golf and not great golf. I've made a lot of mistakes as well.”

His opening 75, which began with a tee shot out of bounds at the first, caused him to lose confidence quickly and while he appeared to be back to form after a second round 67, he had three double bogeys in a 75 in difficult conditions on Saturday and another yesterday when try to be too cute with a bunker shot at the 14th. 

He’s still confident his good golf is good enough to win anywhere and feels good about his chances as he heads to San Antonio in Texas this week to defend his WGC-Dell Match Play title.

“It’s a nice way to round off the week,” he said of the 3-3-3 or eagle-par-birdie finish that made up for the double bogey five at the par-three 14th where he took two to escape from sand after short-siding himself.

“I was really disappointed coming off the course yesterday after feeling like my game was in really good shape coming here — you’ve seen glimpses of that this week with a 67 which could have been a lot better on Friday and then a 65 today. I just need to eradicate my mistakes.”

McIlroy’s form has been up and down all season and while he has broken par in 15 of his 22 rounds this year, six over his over par scores have come in his last 11 rounds.

“It’s all about Augusta,” he said of his thoughts heading to San Antonio for the WGC-Dell Match Play. “Next week is a bit of a weird week for my final competitive week going into Augusta but it will be great to get some matches under my belt.”