Los Angeles - 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 Match Play in Texas.
Day advanced with a 3 and 2 victory over Brooks Koepka.
The victory means the Australian will regain the world number one ranking from American Jordan Spieth, who was ousted 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.
"I was a little annoyed going from the ninth green to the 10th -- letting Chris get back into it with a three-putt on nine," McIlroy said.
"But after that I was four-under for my next six holes and that was enough to get the job done."
McIlroy, who edged Zach Johnson 1 up to reach the quarters, admitted he has more than a title repeat on his mind.
"It's great for my confidence, especially with the Masters only a couple of weeks away," he said. "To get a good run in this tournament, which is my last competitive start before Augusta is really pleasing."
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.
Day, who had downed Brandt Snedeker 3 and 2 on Saturday morning, said he expected the clash with McIlroy to be "a lot of fun" although he's hoping the weather will be warm for the benefit of his injured back.
"It's been a bit of an up and down week, especially emotionally," said Day, whose back was so bad after his first-round win on Wednesday he thought he might have to withdraw.
"It's been quite amazing to be able to start the week and not really thinking I'm going to be able to play with what happened Wednesday. And turn around I'm number one in the world on Saturday.
"It's a strange feeling," he said. "I'm glad I stuck it out."
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.
Cabrera 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 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.