New York, Sep 14 (DPA) Roger Federer chased down a Novak Djokovic lob and fired a back-facing, between-the-legs, cross-court winner to setup three match points Sunday on his way into a sixth consecutive final at the US Open.

The Swiss superstar, who joked that his effort could rank among the best shots of his career, will play Argentine Juan Del Potro Monday in a rain-delayed final.

Federer spanked Djokovic 7-6 (7-3), 7-5, 7-5 to reach a record 21st career Grand Slam title match.

Del Potro denied injured Rafael Nadal a place in a career-first New York final, reaching his own first title contest at a major with a 6-2, 6-2, 6-2 victory over the Spaniard.

'It was a very difficult match today,' said Nadal, who is suffering with an abdominal muscle pull.

'He played really well today, much better than me. In the first two sets, I had lots of chances to keep the score more tight. But I could do nothing today. I congratulate him.'

Federer, the 15-time Grand Slam champion, even amazed himself with his closing effort against Djokovic, whom he's now beaten in nine of 13 meetings.

'I was in a difficult position. I had nothing to lose,' he said after the win in just over two and a half hours. 'We try to practice that shot, but it never works. Maybe it's the greatest shot I ever hit in my life. It's unbelievable.'

The semi-final was 'an unbelievable match,' Federer said. 'I had a lot of fun today. Novak had a great event himself, so it was good to come through because he was tough.