Patrick McEnroe overcame a pulled back muscle and a tenacious Italian with a nickname from a martial-arts movie to become the most improbable semifinalist at the Australian Open Wednesday.

It was a five-set copy of his better-known brother's Grand Slam start, and it put him in some select company.Unseeded and ranked 114th in the world, McEnroe was in dull, sometime stabbing pain after injuring his lower left back while serving in the second set against Italy's Cristiano Caratti.

The "Caratti Kid," No. 101 in the rankings, smelled blood and came back from two sets down, but McEnroe - with the help of a handful of winners - held on to win on his fifth match point, 7-6, 6-3, 4-6, 4-6, 6-2.

McEnroe will be the decided underdog in the men's final four Friday. He'll play second-seeded Boris Becker, who beat 10th-seeded Guy Forget 6-2, 7-6, 6-3.

The other semifinal has top-seeded Stefan Edberg against two-time defending champion Ivan Lendl. Edberg beat Jamie Yzaga 6-2, 6-3, 6-2, before Lendl defeated Goran Prpic 6-0, 7-6, 7-6. Lendl won last year when Edberg retired in the final with a stomach-muscle pull.

"It's just like you all expected - Edberg, Lendl, McEnroe and Becker," Patrick said after raising his five-set record to 3-0. "It's an unbelievable opportunity. Obviously, I'm thrilled about it."

McEnroe, long known as a doubles star, pumped his fists in relief and elation as Caratti's final backhand hit the net. The two exchanged high-fives and McEnroe collapsed into his chair, tears streaming down his cheeks.

"I felt like, `Wow, I did it,"' he said.

He left center court to a standing ovation from the sparse crowd, throwing a couple of sweatbands to his fans as walked off.

"It (the back) feels a little better now," McEnroe said about a hour after the match. "It bothered me off and on. It affected some shots, particularly my serve."

Tournament doctor David Bolzonello said McEnroe received heat, massage and ice treatment for the injury after the match and was given an anti-inflammatory drug. He said the injury could force a postponement of his men's doubles semifinal scheduled for Thursday.

This is McEnroe's second Grand Slam singles appearance, after winning his only Slam title in men's doubles at the French Open in 1989. His oldest brother, John, also reached the semifinals in his second Grand Slam singles outing, Wimbledon in 1977, after winning the French Open mixed doubles with Mary Carillo a month earlier.

It also matches John's best Aussie performance: a trip to the semifinals in 1983. Patrick's win also made the McEnroes the first brothers to reach Grand Slam semifinals in the open era.

John McEnroe, who was defaulted for arguing line calls at last year's Australian Open, skipped this year's tournament with injuries. Kid brother is carrying on the family tradition that includes 17 Grand Slam titles - all but one of them John's.