LOS ANGELES (AFP) - Newly-minted world number one Jason Day defeated defending champion Rory McIlroy 1 up on Sunday to reach the final of the World Golf Championships Match Play in Austin, Texas.

Day triumphed in a heavyweight battle of the past two WGC Match Play champions and will duel with Louis Oosthuizen in Sunday afternoon's championship match.

The South African beat Rafa Cabrera Bello 4 and 3 in the other semi-final.

Day's quarter-final victory over Brooks Koepka on Saturday, coupled with Jordan Spieth's round of 16 defeat, ensured the Australian will regain the world number one ranking from Spieth on Monday.

Day, the 2014 champion, can join Tiger Woods and Geoff Ogilvy as multiple Match Play winners.

He ended McIlroy's bid to join Woods as the only player to successfully defend the title.

Oosthuizen is through to the final for the first time after falling in the quarter-finals each of the past two years. He has yet to play the 18th hole this week.

Cabrera Bello, who will move into the top 50 in the world next week and thereby gain entry to the Masters, didn't look as sharp as he had all week. Oosthuizen took the lead with a birdie at the first and never relinquished it, winning the 14th and 15th to close it out as Cabrera Bello bogeyed both.

"I could see he was not playing well," Oosthuizen said. "So I just wanted to hit greens, put pressure on him and make sure I made pars."