Jordan Spieth lost in the fourth round at the Dell Match Play in Austin, Texas. Then, he lost his No. 1 ranking.

Jason Day assured a return to the top of the rankings when he beat Brooks Koepka, 3 and 2, to advance to the semifinals, which seemed unlikely Wednesday after his back seized up at the end of a 3-and-2 victory over Graeme McDowell in the opening round.

“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,’’ Day said. “And turn around I’m No. 1 in the world on Saturday. I’m glad I stuck it out.’’

Next up for Day is defending champion Rory McIlroy, who overwhelmed Chris Kirk on the back nine at Austin Country Club to close out the match, 4 and 3, running his unbeaten streak in this tournament to 12.

Louis Oosthuizen, who took the lead on the fourth hole and never trailed in his 3-and-2 elimination of Spieth, came back in the afternoon and defeated Dustin Johnson, 2 and 1, to reach the semifinals.

“I was definitely the underdog [against Spieth],’’ said Oosthuizen, who will face Spaniard Rafa Cabrera Bello, a 2-and-1 winner over Ryan Moore. “I just wanted to go out and play solid golf.’’

Spieth’s unraveling occurred when he showed up at the practice range, developed a slice, and scrambled to make a fix that didn’t work.

PGA — Ian Poulter, who failed to qualify for the Match Play, shot a 4-under 68 on a windy Coco Beach to take the third-round lead in the Puerto Rico Open.

LPGA — Top-ranked Lydia Ko made three straight birdies early on the back nine and finished with a 5-under 67 to take a three-stroke lead in the third round of the Kia Classic in Carlsbad, Calif.