* Wawrinka wins first grand slam title
* Nadal hampered by back injury from second set
MELBOURNE, Jan 26 (Reuters) - Stanislas Wawrinka staged the biggest upset in a grand slam final for nearly half a decade when he put down a wounded Rafa Nadal 6-3 6-2 3-6 6-3 in a surreal Australian Open final to win his maiden major title on Sunday.
The record books will record a shock on a par with Juan Martin del Potro's victory over five-times defending champion Roger Federer at the 2009 U.S. Open but it was a far more complex evening than that.
The Swiss had started brilliantly to take the opening set of a match few had given him a chance of winning but the contest changed irrevocably when world number one Nadal pulled up with a back injury at the beginning of the second stanza.
After medical treatment, the Spaniard remarkably came back to win the third set and it took some time before the eighth seed was able to sufficiently regather himself to serve out for victory after 141 extraordinary minutes.
Wawrinka sat on court in tears after receiving the trophy from Pete Sampras, who, if the pre-match script had held, would have handed it to Nadal after the Spaniard matched his haul of 14 grand slam titles.
"It's quite crazy what's happening right now," said Wawrinka, the first player since Del Potro to break the grand slam monopoly of Nadal, Federer, Novak Djokovic and Andy Murray.
"I never expected to win a grand slam. I never dreamed about that because for me, I was not good enough to beat those guys."
Nadal, top seed and holder of the French and U.S. Open titles after a brilliant 2013 season, did everything in his power to avoid taking the limelight away from Wawrinka's arrival into the grand slam winner's circle.
"That's not the real moment to talk about (the injury)," said Nadal, who admitted his back had troubled him from the warm-up.
"It's the moment to congratulate Stan. He's playing unbelievably. He really deserved to win that title. I'm very happy for him. He's a great, great guy. He's a good friend of mine. I am really happy for him.
"So just congratulate him for everything."
His tears, though, told of the frustration of an ultra-competitive man whose path to a second Australian Open title had been blocked by injury for a fourth time in five years since his sole triumph in 2009.
"IMPOSSIBLE TO WIN"
The 27-year-old retired in his quarter-final against Murray in 2010, slumped out while clearly injured against compatriot David Ferrer in the 2011 last eight and missed the entire tournament in 2013.
"I tried hard until the end, trying to finish the match as good as I could for the crowd, for the opponent, for me," Nadal added.
"So that's what I did, tried everything until the last moment, but it was impossible to win this way."
After also beating defending champion Djokovic in the quarter-finals, Wawrinka became first man to defeat the top two seeds at a grand slam since Spain's Sergi Bruguera at Roland Garros in Paris in 1993.
Wawrinka will now move above Federer in the world rankings to become Swiss number one, an almost inconceivable thought not more than 12 months ago when he was considered no more than a decent top 20 journeyman.
The 28-year-old has been a revelation under new coach Magnus Norman over the last six months, though.
Serving like a dream and smashing winners from both wings, he grabbed a break for 3-1 when Nadal netted a backhand and held firm to hold his own serve and take his first ever set off the Spaniard in their 13 meetings.
Nadal had been glancing nervously up at his uncle and coach Toni in the stands in the first set but it was not until he had been broken to love to start the second that it became apparent that something was wrong.
When he came to serve again at 2-0 down, he suddenly pulled up short and at the next change of ends, raced off court for a medical timeout.
Wawrinka raged at umpire Carlos Ramos for not explaining clearly what was going on and the crowd booed the 13-times grand slam champion when he returned to court, obviously suspecting some gamesmanship to arrest his slide.
It soon became obvious that was not the case as, clearly impeded, Nadal shuffled lamely along the baseline.
For the remainder of the set the crowd were like rubber-neckers at a car crash, watching to see how long Nadal could last before retiring.
After winning the second set, though, Wawrinka appeared to have been knocked off his stride by Nadal's predicament and whatever treatment the Spaniard had received kicked in and he incredibly won the third.
"It wasn't easy," said Wawrinka. "He got injured. I saw that. He wasn't serving at all. He wasn't moving during one set. Then was a completely different match. I had to focus on myself, to try to find the way just to win it."
Wawrinka grabbed the first break in the fourth set with a thumping forehand winner only for Nadal to break right back but another huge forehand soon had the world number eight serving for the match at 5-3.
By now he could scent victory and, after punching himself in the head a couple of times, he served out to love, marching up to the net to slap another forehand past Nadal to deliver the coup de grace.
"I was unhappy because normally that's not the way I want to win the match," Wawrinka said.
"But it's a final. At the end I won in four sets. I think I finished the match well. To get the win, it's just amazing for me." (Editing by Justin Palmer)