Jason Day defeats Rory McIlroy in semifinals, Louis Oosthuizen in finals to win Dell Match Play

Jason Day made his return to No. 1 in the world feel even sweeter Sunday by winning the Dell Match Play.

Day outlasted Rory McIlroy in an epic battle in the morning semifinals by making a 12-foot par putt on the 18th hole. The championship match against Louis Oosthuizen turned into a victory lap at Austin (Texas) Country Club.

And to think Day thought about pulling out Thursday morning when his back seized up on him. Three days later, he looked better than ever.

“It’s been a memorable week, not only to win the Dell Match Play but to get back to No. 1 in the world,” Day said.

Day pulled ahead with a 10-foot birdie putt on the par-three fourth hole, stretched his lead to 3 up at the turn and was relentless with his power and short game the rest of the way. He closed out Oosthuizen with a wedge to three feet for a conceded birdie on the 14th hole and a 5-and-4 victory.

It was the largest margin in the championship match since Tiger Woods beat Stewart Cink, 8 and 7, in 2008 in a 36-hole match in Arizona.

Day was assured of the No. 1 ranking when Jordan Spieth lost Saturday morning in the fourth round and Day won his quarterfinal match that afternoon. But he said it would have felt hollow without leaving town with the trophy.

Oosthuizen, who knocked out Spieth, won the opening hole with a par and that was it. His only birdie was an eight-foot putt on No. 5 after Day had stuffed his wedge from the rough into two feet.

“I couldn’t buy a birdie,” Oosthuizen said. “I played OK, but nothing spectacular. Up against a guy like Jason, you need to play three or four under to have a chance.”

Day joined Woods and Geoff Ogilvy as the only multiple winners of the Match Play. Along with replacing Spieth at No. 1 in the world, the 28-year-old Australian figures to be the favorite when he goes to Augusta National next week to start preparing for the Masters on April 7-10.

Day, coming off a victory last week in the Arnold Palmer Invitational, has won six of his last 13 tournaments dating to the PGA Championship.

None of this looked possible when he winced and grabbed his back on the final hole he played in his 3-and-2 victory over Graeme McDowell on Wednesday. Day said he thought about going back to Ohio for tests, especially with the Masters around the corner. He went through therapy every day, and stuck it out. And while his back got progressively better, so did his game.

Day played only 101 holes over seven matches — one match was six holes when Paul Casey withdrew with a stomach ailment — and only had one match go the distance.

Rafa Cabrera Bello of Spain, who lost to Oosthuizen in the other semifinal, won the last three holes to beat McIlroy in the consolation match. The real consolation for the Spaniard was moving into the top 50 in the world ranking to earn a trip to the Masters for the first time.

Day might have won this tournament in the morning.

His semifinal match with McIlroy — only the second semifinal in the 18-year history of this event that the Nos. 2 and 3 seeds met — lived up to the expectation. The lead changed seven times. They combined for 11 birdies in chilly, blustery conditions.

Ko affirms ranking

Lydia Ko won the Kia Classic in Carlsbad, reaffirming her position as the top player in the world heading into the first major championship of the season.

Ko birdied the final three holes for her third straight five-under 67 and a four-stroke victory over second-ranked Inbee Park.

A week after finishing second in Phoenix in the Founders Cup, Ko headed to Rancho Mirage for the ANA Inspiration with her first LPGA Tour victory of the year and 11th overall. The 18-year-old New Zealander also won the Ladies European Tour’s New Zealand Women’s Open in February. She finished at 19-under 269.

Finau gets first win

Tony Finau won the Puerto Rico Open for his first PGA Tour title, beating Steve Marino with a birdie on the third hole of a playoff at windy Coco Beach in Rio Grande.

The 26-year-old Finau won with his third straight birdie in the playoff on the par-five 18th, blasting out of the back bunker to three feet. Marino was just off the back edge of the green in two, but left his putt from the fringe 4 1/2 feet short and missed his birdie try to the right.

Finau and Marino each shot two-under 70 to finish at 12-under 276. Marino birdied the 18th to tie Finau, and got into the playoff when Finau missed a six-foot birdie putt. Finau and Marino each birdied the 18th on the first two playoff holes.

Marino is winless on the PGA Tour. He also lost in a playoff in the 2009 Colonial.

Ian Poulter and Mexico’s Rodolfo Cazaubon finished a stroke out of the playoff.

Poulter closed with a 72, missing an 18-foot birdie putt on 18. Playing the event for the first time after failing to get into the Dell Match Play as the first alternate, the Englishman took a one-stroke lead into the final round.