Jason Day is assured of being world No.1 again.

Australian Jason Day was to replace Jordan Spieth as world No. 1 yesterday after advancing to a mouth-watering WGC-Dell Match Play semifinal 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 quarterfinal by the same score on Saturday at Austin Country Club in Texas.

Northern Irishman McIlroy, the defending champion, followed a 1-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 semifinal.

Day said his return to world No. 1 — 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 No. 1 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."

Four-time 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 US 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 said.

After taking out Spieth, Oosthuizen beat another American Dustin Johnson 2&1 in the last eight.

Cabrera-Bello, who came into the tournament on the back of two runner-up finishes on the European Tour, beat Ryan Moore 2&1 to reach the semis.