Australian Jason Day is assured of being world number one again but he will not be satisfied unless he cements it by taking out the World Golf Championships Match Play.

With 3&2 wins over Brandt Snedeker and Brooks Koepka, Day surged into a mouth-watering and highly anticipated semi-final showdown with world number three Rory McIlroy on Sunday, ensuring he will overtake Jordan Spieth at golf's summit.

But getting the ranking is not enough for the 28-year-old, who wants to solidify it into the Masters and dominate it as long as possible.

"To be able to get back to number one in the world is fantastic. But I wasn't really focusing on it because I know that winning takes care of everything," Day said.

"I've got two more matches to try and win and I'm really trying to focus on that because I really want to win this tournament.

"If I want to stay on top I have to win. It would be really fantastic to be able to go into Augusta as number one in the world. That would be really exciting for me."

If Day finishes fourth in Austin his third stint at the top could be short-lived as he will move just fractions ahead of Spieth and the American is playing the Houston Open next week.

If Day is second or third he will have a better chance of retaining it but if he wins he will assure himself of driving down Magnolia Lane as the game's top dog.

"You've got to embrace it. You've got to enjoy that," Day said.

"I enjoy those moments because it can only make me better. To be able to get that feeling walking around there as number one would be a pretty cool feeling."

To win the event he first must get through McIlroy, the defending champion who has now gone 12 matches without a loss over two years.

Day won the event in 2014, which adds to the magnitude of the semi-final shootout.

"I know Rory is going to be a tough opponent. He always hits it great. It's definitely going to be a difficult match," Day said.

"But I'm excited about the challenge of trying to beat Rory."

'I need any sort of advantage against McIlroy'

Jason Day during the first round of the World Golf Championships-Dell Match Play ( Getty Images: David Cannon )

On Wednesday, a bulging disk in his back almost took Day out of the tournament so the fact he has played 19 holes fewer than McIlroy over the five matches should come in handy.

"Any sort of advantage I can take, I need against him," Day said. "He's obviously a fit-looking bloke, so I don't think he's going to have any trouble with that.

"I've got to make sure I'm sharp and I've got to come out on top and make sure I don't miss any information out there and focus on just giving every shot 100 per cent."

The Northern Irishman, who took out Zach Johnson and Chris Kirk to advance, was also happy with the big time match-up.

"I relish it," McIlroy said. "He's playing really well at the minute, coming off a win last week. I'll be really up for it. It's a big match."

The other semi-final will be between Spain's Rafa Cabrera Bello and South African Louis Oosthuizen

AAP