Now a first English finalist has been guaranteed following Casey's 4 and 3 win over American Sean O'Hair at the desert course near Tucson and Fisher's 2 and 1 victory over US Ryder Cup hero Justin Leonard.
Northern Ireland's McIlroy, in his first tournament as a professional in the United States, could not continue his excellent form, the 19 year old falling to Australia's Geoff Ogilvy 2 and 1 in a tight first match of the day.
"Geoff played incredibly well and I needed to get birdies but it just wasn't enough in the end," McIlroy said.
"But I can take a lot out of this week. I played well and I've held my own against some of the best players in the world so it's been good."
World Number Eight Ogilvy was the Irish teenager's first encounter this week with a top ten player having previously knocked out Louis Oosthuizen, Hunter Mahan and Tim Clark.
McIlroy started in ideal fashion with a birdie at the second to go one up but Ogilvy, the 2006 winner and 2007 runner-up, was soon back in business, winning the fourth hole as his younger rival three putted for bogey and the par four fifth as McIlroy, having produced the longest drive of the week with a 388 yard tee shot, two putted for par.
It was nip and tuck until the 11th when McIlroy found the desert off the tee on the way to a bogey six while the former champion birdied. A birdie at the 13th to win the hole reduced the deficit to one hole and McIlroy fought tooth and nail to stay alive, holing a 12 foot birdie putt at the par three 16th to extend the match, only for Ogilvy to close him out at the 17th.
The Australian will play Stewart Cink later this evening, the American having defeated Ernie Els 2 and 1.
Casey was trying to go one better than his previous best performance in 2007 when he was beaten 6 and 4 by Ogilvy at the same stage and he was helped by O'Hair's waywardness off the tee.
As the American sprayed it around, Casey went six up after ten holes, continuing his run of never having gone behind in a match this week.
There was a brief rally from O'Hair on the back nine and Casey posted his first bogey of the round at the 14th, only his second in three days. Yet it was a minor annoyance as the Englishman claimed victory at the 15th hole.
Casey revealed that O'Hair had been ill going into their match although he was critical of his own play in not closing out the match sooner.
"It's actually kind of tricky," Casey said.
"Halfway through the match you feel you can go for stuff and when you try not to make mistakes, mistakes happen.
"To be honest I think Sean got a little bit of food poisoning last night and he got some breaks when the ball stayed on ridges and could have rolled back. If those had gone the other way it could have been a very close match."
In the last quarter-final to go out, Fisher was two up after 15 holes on Leonard but the American was far from beaten and closed the gap with a birdie two at the 16th.
Leonard piled on the pressure at 17 with an approach shot from 190 yards to eight feet from the hole but Fisher responded brilliantly by also sending his second shot to eight feet from a fairway bunker 178 yards out.
Putting first, Leonard missed, leaving Fisher to hole out to win the match, 2 and 1.
"I’m very relieved," he said.
"I felt I played really, really good for 12 holes. Then hit a slight mishit on 13 and lost that one and then whiffed it on 14.
"I thought come on knuckle down and to win 14 was huge. Then to knock it on 15 and two putt was massive.
"Then all of a sudden I blocked it right on 16. I was thinking trust the swing as I was hitting it great and unfortunately didn’t. 17 was another mystery but I hit a great shot in. To see Justin miss his birdie was surprising but I thought come on and knock it in and fortunately I did."
