World Number 1 Jason Day has secured his second World Golf Championships-Dell Match Play victory after defeating South African Louis Oosthuizen five up in the final round.

Day progressed to the final round after a one up win over defending champion Rory McIlroy.

"Rory was difficult because from tee to green he doesn’t miss any shots. Even if he does miss one slightly, he’s always back on top of it and hitting a great shot the next time. But I think if he had holed a few more putts early on, it would have been very difficult to catch him," said Day.

"All I wanted to do, if I didn’t hit it as good as him, was to frustrate him with my short game. If I missed the green I wanted to get up and down to 20, 30 feet and hole the putt, that was my main goal. We had a really, really, good match." 

Day took the early lead with a win at the opening hole following a birdie before losing the advantage at the second hole when he hit his approach into the hazard.

The pair traded birdies for wins and losses in a back-and-forth match for several holes and were all square playing the par 12th hole when Day hit a shot from 267 yards that barely cleared the hazard to find the green, resulting in a birdie and a 1 up lead.

A halved hole at the 17th sent the match to the 18th with McIlroy needing to win the hole to extend the match, but both players missed the green in regulation. Day chipped to 13 feet and holed his putt to win and advance to the Championship Match and faced his 2015 Presidents Cup teammate, Oosthhuizen, in the Championship Match.

Oosthhuizen started strongly, going one up with par on his opening hole giving him a narrow lead which he maintained until the pair were all square by the par-4 third hole. A birdie on the 4th gave the Aussie the spark he needed and by the par-3 7th Day turned up the heat on the South African going two up, which was extended to three at the turn. Back-to-back birdies on the 13th and 14th sealed victory for Day and his sixth win from his last 13 starts.

"It’s been an amazing week; I’ve really enjoyed this golf course. I’ve not only been able to utilised the great short game that I had from last week to this week, but also play the way I did from tee to green. It’s been really fantastic," said Day.

"It’s tough. You’re playing against seven guys not a field, so you don’t have to worry about so many other guys that you’re normally playing against. You have to know when and when not to go for things, if someone makes a mistake you have to be able to aim it 20 foot from the pin and be able to hole that putt. It’s all in the momentum."

This was the second time Day and Oosthuizen have met in a match during the Dell Match Play. Day defeated Oosthuizen in the quarterfinals, two and one, in 2014. Day then went on to win the title that year following wins against Rickie Fowler in the semifinals and Victor Dubuisson in the final.

Day’s victory over Brooks Koepka in round five saw the Australian return to World Number 1. Day becomes the second Australian winner, and third multiple winner, of the Dell Match Play after Tiger Woods won in 2003, 2004 and 2008 while Geoff Ogilvy was victorious in 2006 and 2009.