World number one Novak Djokovic saved two match points against Roger Federer for the second year running to win a thrilling US Open semi-final.
Federer's attacking style held sway early on but Djokovic roared back to win 6-7 (7-9) 4-6 6-3 6-2 7-5.
The Serbian now stands one win from a fourth Grand Slam title, and his third of an incredible 2011 season.
He will play Rafael Nadal on Monday in a repeat of last year's final after the champion beat Britain's Andy Murray.
"It was definitely the biggest win of this year, one of the biggest wins of the career, under the circumstances," said Djokovic. "I was two sets down. Roger was in control, he was playing better and then I switched gears and I managed to play much better in three sets."
Djokovic beat Federer in a dramatic semi-final 12 months ago before Federer took some measure of revenge by ending the Serbian's unbeaten run at the French Open in June.
After a rain delay of almost an hour and a half, the latest chapter of their rivalry finally got under way on a packed Arthur Ashe Stadium and there were no break points in a desperately tight first set.
A tie-break always looked like likely and it was Djokovic who faltered with an error and a double-fault to fall 5-2 behind, although Federer would need five set points before clinching it.
With the tension of the tie-break out of the way, Federer stepped up a gear and got the first break of the match at 1-1 thanks to a beautiful backhand up the line and a wrong-footing forehand.
There had been no real alarms for the Swiss until he faced break point in game six, and a wayward forehand brought Djokovic back into the set, but minutes later Federer had broken to love to move 4-3 ahead.
It was a huge blow to Djokovic, who trooped back to his chair disconsolately and found himself two sets down minutes later.
The Serbian's form had been a shadow of that which brought him into the match with a stunning 62-2 record for the year, but he grabbed a lifeline when Federer netted a backhand on break point early in the third.
Djokovic served his way to the third set and made a major breakthrough by breaking at the start of the fourth with a brilliant backhand from out wide that Federer could only volley into the net.
The world number one was suddenly in total command, racing through the fourth set for the loss of just three points on serve to force a decider.
Roared on by the majority of the 23,000 spectators, Federer regained the initiative with a vital break at 4-3 in the fifth set when Djokovic dropped serve to love, and serving for the match at 40-15 he looked as good as home.
But a breathtaking Djokovic return, that the Serbian celebrated with the crowd, and a net cord saw those chances slip away, and after the 30-year-old Swiss double-faulted to give the break back there was no stopping the top seed.
Djokovic powered through the final four games of the match to claim his 63rd, and surely most dramatic, victory of 2011.
"Sure, it's disappointing, but I have only myself to blame," said Federer, who will end the year without a Grand Slam title for the first time since 2002.
"I set it all up perfect, but I couldn't finish it. It was a tough year in terms of some tough losses at some crucial stages of the season but look, it's not the first time it's happened.
"I have had big matches where I ended up losing some, but the majority I was able to win throughout my career."
Asked about his celebration after saving the first match point, Djokovic said: "I needed to get some energy from the crowd. It was normal to expect they were a little bit more on the side of Roger, maybe the greatest player ever. It was no surprise.
"But I thought at that certain moment I had to get some of the crowd energy behind me. And it worked."