Watch: Del Potro knocks out Federer from US Open

Roger Federer conceded he was not in the right shape to challenge for another grand slam title as the elusive US Open clash with Rafael Nadal slipped away again.

After the Spaniard defeated Andrey Rublev, it seemed virtually nailed on that New York would witness a first meeting between the great beasts of men's tennis in the semi-finals.

Juan Martin del Potro, sick and exhausted after his epic comeback to beat Dominic Thiem on Monday, surely would not have it in him to beat the Australian Open and Wimbledon champion.

But the Argentinian again showed what a huge heart he possesses, saving four set points in the third set tie-break and going on to win 7-5 3-6 7-6 (10/8) 6-4.

This is the sixth time Federer and Nadal have come within one match of meeting in New York but the missed opportunity was not uppermost in Federer's mind.

The 36-year-old insisted it was his physical struggles, along with the form of Del Potro, that decided the match.

Federer arrived in New York nursing a back problem but appeared to have put the doubts behind him after surviving two five-set matches in the opening two rounds.

Asked if he was thinking about Nadal, the third seed said: "I wasn't. You were. It's...