Jason Day marked his return to the top of the world rankings in emphatic fashion with a second WGC-Dell Match Play title in three years as Rory McIlroy turned his attention to completing a career Grand Slam.

Day, who replaces Jordan Spieth as world number one on Monday by virtue of his performance at Austin Country Club, beat defending champion McIlroy by one hole in a hard-fought semi-final on Sunday.

And the 28-year-old Australian then cruised to a 5&4 victory in the final against former British Open champion Louis Oosthuizen, who had earlier defeated Rafael Cabrera-Bello 4&3 in the last four. The Spaniard went on to beat McIlroy 3&2 to finish third.

“I’m very, very thrilled,” Day said at the presentation ceremony. “It’s been a memorable week this week, not only winning the Dell Match Play championship but also to get back to number one in the world.

“I have really enjoyed this golf course. I have been able to utilise the great short game that I had from last week but also to play the way I did from tee to green and hit the clutch shots.”

Oosthuizen won the opening hole after driving the green on the short par four, but lost the third after finding a hazard with his second shot and went behind for the first time when Day birdied the fourth.

A bogey on the seventh saw the South African fall further behind and Day turned the screw with a birdie from six feet on the ninth to close in on a second win in succession, following his victory in the Arnold Palmer Invitational last week.

Day, who feared he would be unable to continue in the event after injuring his back on Wednesday, moved further ahead with a birdie on the 13th and was three feet away from another on the 14th when Oosthuizen bowed to the inevitable and conceded.

“It was 50/50 when I woke up on Thursday and was hitting balls on the range,” Day added. “My team wanted me to get an MRI on my back but I thought if I have to pull out I can get an MRI back home. I’m very pleased with how it recovered so quickly.”

McIlroy has never successfully defended a title in his professional career but had made no secret of wanting a win under his belt before attempting to join Gene Sarazen, Ben Hogan, Gary Player, Jack Nicklaus and Tiger Woods in completing the career Grand Slam at the Masters.

And the 26-year-old felt his excellent match-play record gave him a great chance of following the likes of Day, Spieth, Bubba Watson, Adam Scott and Charl Schwartzel into the winner's circle this year in his final competitive outing before heading to Augusta.

However, after twice edging ahead on the front nine with birdies on the sixth and eighth, his semi-final with Day was effectively decided in the space of four holes at the start of the back nine.

Day got back on level terms with a birdie on the 10th and then crucially saved par from left of the green on the 11th, before McIlroy missed his birdie attempt from five feet.

The US PGA champion then birdied the next two holes – his approach to the par-five 12th narrowly clearing the water surrounding the green – as McIlroy duffed his chip to the 13th to fall two behind for the first time.

McIlroy, who would have equalled Tiger Woods’s record of 13 matches without defeat in the event with a win, did reduce his deficit with a birdie on the 14th, but after both players birdied the 16th, Day produced a brilliant par save on the last to seal victory.

A disappointed McIlroy told Sky Sports: “I thought it was a good quality match. If I have to look back on it there were a couple of opportunities around the third and fourth where I did not convert, and then not birdieing 12 or 13 was the real downfall.

“Losing 12 and 13, I think the catalyst for that was that shift in momentum on 11 and I never really recovered. To go two down I knew I had an uphill battle on my hands and Jason was not giving me anything.”

On the positive front, McIlroy added: “I was saying coming into this week that no matter if I play three, six or seven rounds, playing match play and feeling like you are in contention every time you step on the first tee is a great feeling and something you can’t replicate in practice.

“The week has been very positive overall. I wish I would have been able to get one more win under my belt, but I am very happy with my game going into Augusta.”