(Reuters) - Australian Jason Day will replace Jordan Spieth as world number one on Sunday after advancing to a mouth-watering WGC-Dell Match Play semi-final against Rory McIlroy, while the Texan was beaten in the last 16 by Louis Oosthuizen.
Day, who has been bothered by a back problem, beat Brandt Snedeker 3&2 in the last 16 and then Brooks Koepka in the quarter-final by the same score on Saturday at Austin Country Club.
Northern Irishman McIlroy, the defending champion, followed a one-up win over British Open winner Zach Johnson by defeating Chris Kirk 4&3 to set up the first ever match play encounter between the pair.
South African Oosthuizen, the 2010 British Open winner, will face Spain's Rafa Cabrera-Bello in the other semi-final.
Day said his return to world number one -- he occupied top spot for four weeks late last year -- was reward for all the hard work he has been putting in.
"It's not so much the number one ranking that gets me excited, it's more the journey and the process," he said.
"It's that delayed gratification I'm really thankful for because I have been busting my butt.
"Tomorrow I know I've got a really tough one. My back is a little shot right now."
Four-times major winner McIlroy is bidding to become the second player to retain the Match Play title after Tiger Woods won it in 2003 and 2004.
"My record in match play as a pro is pretty good and it was pretty good as an amateur as well," he said. "To get to the last day for the second year in a row is great for your confidence.
"Especially with the U.S. Masters only a couple of weeks away, to get a good run in this tournament, which is my last competitive start before Augusta, is really pleasing."
An out-of-sorts Spieth, who will be defending champion at Augusta, battled his swing from the moment he stepped on to the range to warm up and lost 4&2 to Oosthuizen.
"I had great ball control the last three days, played very, very solid rounds, and I got to the range this morning and I was hitting some slices with my irons, which is bizarre," the 22-year-old told Golf Channel.
After taking out Spieth, Oosthuizen beat Dustin Johnson 2&1 in the last eight and said a reprieve at the par-five 12th had been the key to his win over the long-hitting American.
"He gave me a bit of a chance on 12 when I knocked it in the water and he followed me in there. That was a bit of a save for me," said Oosthuizen.
Spaniard Cabrera-Bello, who came into the tournament on the back of two runner-up finishes on the European Tour in the Middle East, beat Ryan Moore 2&1 to reach the semis.
"I enjoy a lot match play and feeling confident this week," said the 52nd seed. "I’m really proud to be in the semi finals."
(Reporting by Andrew Both; Editing by Peter Rutherford)