Australian Jason Day beat an ailing Louis Oosthuizen to move into the semi-finals of the WGC-Accenture Match Play Championship on Saturday, as Graeme McDowell enjoyed a unique experience at Dove Mountain.

Oosthuizen received lengthy treatment for a long-standing back injury before play started and needed more out on the course before eventually losing 2&1 to Day, who reached the last four for the second year running.

Former Open champion Oosthuizen looked to be in real trouble from the outset as he carved his opening tee shot into the desert. After attempting to hack clear from a bush, Oosthuizen quickly conceded the hole and seemed likely to fall two down after another errant drive on the second.

However, this time the South African took a penalty drop and then hit a superb third shot with a fairway wood on to the green, holing the putt from 20 feet for an unlikely winning birdie.

Another birdie on the third briefly took Oosthuizen one up, but he made a mess of the fourth with two bad pitches and went behind to a Day birdie on the fifth.

Day won the ninth and 11th to move three up, but Oosthuizen continued to battle hard, winning the 13th and halving the 14th and 15th with birdies, but Day held on to seal victory.

McDowell had played 58 holes to reach the quarter-finals and had remarkably never stood on any tee with the lead, producing miraculous comebacks to edge past Gary Woodland, Hideki Matsuyama and Hunter Mahan.

But the former US Open champion needed just two holes to finally get his nose in front against Victor Dubuisson, a conceded eagle on the par-five second taking him one up and also leaving the Frenchman facing a deficit for the first time this week.

McDowell went further ahead with a birdie on the third, but Dubuisson birdied the fourth and seventh to get back on level terms.

Elsewhere, Jim Furyk was producing a brilliant turnaround in his match with Rickie Fowler, who had looked like progressing to the last four when he went three up on the 11th.

However, Furyk then birdied the 13th, 14th and 15th and won the 16th with a par to move one ahead with two to play.

In the remaining match, Ernie Els was one up after 10 against rising star Jordan Spieth.

Just when Furyk looked on course for the semi-finals, the former US Open champion made a mess of the 17th to allow Fowler back on level terms and then fluffed a chip from short of the 18th green to gift the match to his opponent.

Fowler will take on Day in Sunday morning's semi-final, who admitted he was wary of Oosthuizen despite the obvious injury problem.

Day told Sky Sports: "It's that old saying, beware of the wounded golfer. I think he went out the last four days with no expectations and I think that's why he's playing so great. I wanted to play good myself and get through to the next day.

"I love the golf course and I just like that one-on-one match. It's fun fighting it out to see who's best."

Oosthuizen added: "I was disappointed to start on the range this morning not feeling too good. It got better, the last four or five holes I felt I could get through the ball better. It's a bit painful that it always happens when I'm playing well and I need to address it big time and get it sorted.

"I felt I played really well but you can't give away holes to a guy like Jason. I had a tough start and it was really tough to get back from that."

Dubuisson chipped in for a birdie on the ninth to take the lead for the first time and, after McDowell had won the 11th with a birdie, also birdied the 12th to edge in front again.

However, a poor pitch to the 13th led to a bogey six and allowed McDowell to get back on level terms as another close finish looked on the cards.

In contrast, Els won the 12th and 14th to move three ahead with four to play against a demoralised Spieth.

Els duly completed victory over Spieth on the 16th to set up a semi-final with either McDowell or Dubuisson, with the Frenchman edging ahead once more when McDowell three-putted the 16th for the second day running.