NEW YORK • The scowl on Rafael Nadal's face and his low mutterings directed towards his coaches in the stands were the first indications on Friday night that something was wrong.
When he asked for a trainer to tape his right knee, the level of concern elevated. And then, in perhaps the clearest sign of all, Nadal, one of the greatest ball-hunters in the history of tennis, allowed one to whiz past him without even giving chase.
But in the most unusual indication of his distress, Nadal actually told the chair umpire, in a moment of pique over a questionable call in the second set, that he was going to retire from the match.
And a few games later, he did, putting an end to what had developed into a disappointing semi-final match at the US Open.
After spending more than 17 gruelling hours on court throughout the tournament, Nadal's 32-year-old body - specifically his chronically injured right knee - gave way.
The beneficiary was the third-seeded Juan Martin del Potro, who advanced to his first Grand Slam final since he upset Roger Federer to win the US Open in 2009.
Nadal, the No. 1 seed, lost the first set, 7-6 (7-3), and the second, 6-2, before packing it in. He took off his headband and wristbands, and went over to del Potro to explain his decision. They hugged, and Nadal walked briskly off the court.
Novak has won Wimbledon already. He's playing so good. But when I played Roger nine years ago, he was the favourite to win as well. I will try to make the surprise again.
JUAN MARTIN DEL POTRO, on how he can upset the odds just like he did against Roger Federer to win the US Open in 2009.
Later, he explained that he felt the pain in the fifth game of the first set and that, by the second set, he could not move well enough to be competitive.
"That was not a tennis match at the end, no?" the Spaniard said.
"It was just one player playing, the other one staying on the other side of the court."
It was a cruel fate for the defending champion, who beat South Africa's Kevin Anderson in the final a year ago and was aiming for his 18th Grand Slam title.
Del Potro is also familiar with the feeling of losing out on trophies because of injuries. The 29-year-old has endured multiple wrist operations after winning his only Major, but he is finally getting back to his form of nearly a decade ago.
While Novak Djokovic, his opponent in today's final, holds a 14-4 career record over del Potro, and has won six of their last seven matches dating back to the 2013 Wimbledon semi-final, the Argentinian reminded reporters that Federer was also a heavy favourite in 2009.
"It will be a difficult match because we are close friends," said the world No. 3. "Novak has won Wimbledon already. He's playing so good. But when I played Roger nine years ago, he was the favourite to win as well. I will try to make the surprise again."
Djokovic also seemed to sense that del Potro would be the fan favourite at the Arthur Ashe Stadium today.
"He's a gentle giant. He's very tall, has a big game, but at the same time, he nurtures the right values in life. He cares about his family. He cares about his friends," the Serb said.
"People can relate to that and appreciate what he brings to tennis. We all felt for his struggles with injuries, but he was always a top-five player in the eyes of everyone."
In his lengthy efforts to fully recover from his wrist injuries, del Potro nearly gave up tennis in 2015.
But he said the support of his friends from his hometown of Tandil - many of whom sang and cheered loudly from a suite during the semi-final match - and the expertise of his doctors pulled him through his depression.
"That was the bad moment for me," del Potro said.
"But that is completely in the past, and now, I'm having a good present, looking forward for the future. I didn't expect to get this kind of emotions playing tennis again.
"Reaching finals, winning titles, having my highest ranking, in this moment, everything is almost perfect."
AGENCE FRANCE-PRESSE, NYTIMES
Women's doubles & men's singles final: Singtel TV Ch114 & StarHub Ch208, tomorrow, 1am