* Day beats an ailing Oosthuizen 2&1
* Veteran Els eases past youngster Spieth 4&2
* Fowler edges Furyk one up
* Dubuisson battles past McDowell one up (Adds Dubuisson quotes, detail)
Feb 22 (Reuters) - Jason Day and Ernie Els advanced with relative ease while American Rickie Fowler and Frenchman Victor Dubuisson had to fight hard to reach the WGC-Accenture Match Play Championship semi-finals in Marana, Arizona on Saturday.
Australian Day beat ailing South African Louis Oosthuizen 2&1 to become the first player to make the last four at Dove Mountain while veteran Els progressed with a 4&2 win over an out-of-sorts Jordan Spieth in a match where he never trailed.
Fowler booked his place with a see-sawing, one-up victory over fellow American Jim Furyk, who came from three down after 12 holes to go one up through 16 before running out of steam, and Dubuisson edged Northern Ireland's Graeme McDowell one up.
"Jim started to come on pretty hard there, I just had to stay patient," Fowler, the 53rd seed in a starting field of 64, told CBS Sports. "I forced him to make some birdies. I knew it was going to be tough for him to keep on doing that.
"I just made some good swings coming in. Obviously it's nice to get a win, no matter how it happens."
Day, beaten by eventual champion Matt Kuchar of the United States in last year's semi-finals, will next face fellow 25-year-old Fowler in a battle of 'young guns' while the 23-year-old Dubuisson will take on South African Els, aged 44.
Dubuisson, who had never previously played matchplay golf until his debut this week in a World Golf Championships (WGC) event, rallied from two down after three holes against McDowell with five birdies over the next 12 to wrest control.
"I played well today," said world number 30 Dubuisson, who burst into the limelight by winning his maiden European Tour title at the Turkish Airlines Open in November where Tiger Woods and U.S. Open champion Justin Rose were in the field.
"I knew Graeme was going to be a tough player to beat. I didn't watch him too much. I tried to just focus on my game and I try to make birdies."
McDowell, who had delivered Houdini-like escapes to win his first three matches, faced a 25-foot birdie putt at the 18th to force extra holes but his attempt slid past the left edge of the cup allowing the Frenchman to advance.
Day had trailed by one after Oosthuizen made a fast start with birdies at the second and third but then upped his game to take charge as his opponent had periodic back treatment from his physiotherapist out on the course.
"We both played well today ... it didn't seem like Louis had a sore back because the way he played was pretty good," Day said after finishing birdie-birdie-par-par.
"We both fought hard until the finish. Hopefully I can do a little better this year and press for that (Match Play title) win."
Fowler, wielding a red-hot putter, birdied three of the first four holes to go three up on Furyk before his opponent briefly cut the deficit to one by winning the sixth and seventh holes.
Birdies at the ninth and 11th got Fowler back to three up but Furyk again rallied, levelling the match with three consecutive birdies from the par-five 13th.
PGA Tour veteran Furyk briefly went one up with a par at the short 16th, where Fowler bogeyed, but then stumbled when he bogeyed the par-four 17th after being bunkered off the tee for the match to reach the 18th all square.
After Fowler had comfortably reached the green in two, Furyk duffed his chip from just short of the putting surface on the way to another bogey as his opponent wrapped up victory with a two-putt par.
Former world number one Els, bidding to become the oldest Match Play champion, produced his best golf of the week as he went one up at the second and never relinquished control to reach the semi-finals for the first time in 13 years.
"I played relatively solid golf," said Els, who won the first of his two U.S. Open titles in 1994 when Spieth was just 10 months old. "I had a pretty nice start, I made a couple of birdies early. I felt like I played OK.
"I got it up and down when I needed to. I am fortunate to go through. Jordan was a bit off."
The semi-finals will be played on Sunday morning with the 18-hole final scheduled to take place later in the day. (Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Larry Fine)