LOS ANGELES (AFP) - Defending champion Rory McIlroy birdied four of the last six holes in a 4-and-3 win over Chris Kirk on Saturday to book a semi-final showdown with Jason Day at the WGC Matchplay in Texas.
Day advanced with a 3-and-2 victory over Brooks Koepka.
The victory means the Australian will regain the world No. 1 ranking from American Jordan Spieth, who was ousted on Saturday morning in the round of 16 by South African Louis Oosthuizen.
McIlroy, seeded third, reached the semi-finals of the elite event for the third time in his career.
He took a quick lead over Kirk with an 11-foot birdie putt at the opening hole, but his three-putt bogey at the ninth let Kirk square the match.
McIlroy responded emphatically, with birdies to win the 10th, 12th, 13th and 15th - all with putts from within 10 feet.
Day, the 2014 winner, had three birdies and an eagle against Koepka. He took the lead with a seven-footer at the third, but Koepka birdied the next two to grab the lead.
The American couldn't find another birdie, however, and Day regained the lead with a seven-foot birdie at the 10th and added an eagle at the 12th before winning the 15th with a par.
The other semi-final pits Spain's Rafael Cabrera Bello against Oosthuizen.
Oosthuizen, seeded 16th, followed up his 4-and-2 win over Spieth with a 2-and-1 triumph over eighth-seeded American Dustin Johnson.
Bello, who advanced to the quarters when South Korea's An Byeong Hun retired with a sore neck while 4 down through 11, beat Ryan Moore 2 and 1 to reach the final four.
The Spaniard, ranked 54th in the world and seeded 52nd, never trailed against Moore, who had just one birdie after producing four on the front nine in a 4-and-3 victory over Patton Kizzire in the morning.
Oosthuizen reached the semi-finals after losing in the quarters the last two years. He trailed Johnson much of their match, but sealed it with a 21-foot birdie putt at No. 17.
But it was Oosthuizen's morning match that made headlines.
Spieth hadn't trailed in three round-robin matches.
"I got to the range this morning and I was hitting some slices with my irons, which is bizarre," Spieth said. "I just tried to compensate somehow on the golf course. I got away with it on the first couple of holes and then it got the best of me."
Spieth, who hadn't made bogey at the first six holes in his first three matches, bogeyed the fourth to fall 1 down.
Oosthuizen held onto the lead as Spieth scrambled through a stretch that included two birdies, two bogeys and a double bogey, with the South African nabbing three birdies in the last five holes to seal it.