Del Potro through to US Open final as Nadal retires with injury

Nine years after his first grand slam final, Juan Martin del Potro reached a second as Rafael Nadal retired with knee problems during their US Open semi-final.

The world number one had spent more nearly 16 hours on court in reaching the last four, averaging more than four hours a match during the last three rounds, and that took its toll here.

Nadal had had his right knee taped during his gruelling third-round clash against Karen Khachanov but insisted afterwards it was not a big issue and he appeared in good shape for his next two matches.

The Spaniard had tape applied here after seven games only to take it off two games later. But he took a medical time-out after three games of the second set and, as he limped around the court, it became increasingly clear that he was in serious trouble.

They had battled for 69 minutes in the first set before Del Potro eventually took it on a tie-break. The Argentinian had twice been a break up, and held two set points at 5-4 only to allow his opponent back in.

But Nadal was unable to take advantage and immediately after the time-out, with the 32-year-old struggling to cover the court, Del Potro broke again.

In the sixth game of the second set, he argued with umpire James Keothavong over a line call, telling the British official: "It's all right, i'm going to retire, but you were not right on this one."

Nadal continued but, after Del Potro won the second set to lead 7-6 (7/3) 6-2, Nadal had one last chat with the trainer, shook his head and offered his hand to his opponent.