Rafael Nadal now has the endurance to go on and lift his third US Open title against Kevin Anderson

The days of the world No 1 wilting in longer matches appear to be behind him after he proves too strong for Juan Martin del Potro.

Rafael Nadal celebrates his semi-final win against Juan Martin del Potro at the US Open. Mike Segar / Reuters

Twelve months ago, as Rafael Nadal waved to fans as he walked off court at Flushing Meadows after losing in the fourth round of the US Open to Lucas Pouille, the gesture felt symbolic.

It felt like a final wave goodbye as the Spaniard's major drought increased to 10, the longest of his career since winning his first grand slam at the 2005 French Open age 19.

Last year Nadal looked to be regaining his full mobility after struggling with knee problems for around four years, but there were other niggles, including a wrist injury, that were still hampering him.

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But another concern was the Spaniard's apparent lack of endurance to go the distance in a grand slam.

He lost to Pouille in five sets, went out to compatriot Fernando Verdasco in five in the first round of the 2016 Australian Open, and lost to Fabio Fognini over five in the 2015 US Open.

It was as though he was succumbing to top-10 players outlasting him, so it was not difficult to concur with the view that Nadal, for various reasons, would not be adding to his 14 majors when he left New York last September.

Fast forward to this week and it has been a spectacular transformation for the 31-year-old Spaniard.

Having started the year at No 9, he is now back at world No 1, and if wins Sunday's US Open final against Kevin Anderson then not only will it give him a 16th major, having already won the French Open in June, it will be the first time in four years that he has won multiple grand slams in a calendar year.

It is not difficult to pin-point what the difference has been: Nadal, for the first time in a very long time, is healthy.

After triumphing over Juan Martin del Potro in Friday's semi-final 4-6, 6-0, 6-3, 6-2, he told reporters it was the fact he felt good physically that pleased him most, even above winning titles and claiming a third US Open crown to follow his successes in 2010 and 2013.

"For me, more than winning grand slams or not - of course, if I win, I will be more happy - but it is about being healthy and feeling well and competitive," he said.

"That's already happened in the whole season. So that's the most important thing for me."

Three of Nadal's matches thus far have gone to four sets after he lost the first, and each time he has bounced back strongly, showing physical strength on court as well as mentally.

Sunday will be Nadal's 65th match of the year. He demonstrated against Del Potro that he appears in no danger of wilting.

He dropped the first set to the Argentine, who had conquered Roger Federer in the previous round, but then hit back strongly, winning nine games in a row as he turned the match on its head.

Yes, Del Potro was visibly tired as the effort that went into coming from two sets down to beat Dominic Thiem and then Federer, but Nadal still had to have the energy himself to capitalise on it.

He did so to ruthless effect. It was hard to believe this was the same Nadal who had been unable to go toe-to-toe with Pouille 12 months ago.

Anderson now stands between Nadal and the icing on the cake of a great renaissance year.

The 31-year-old South African has had to work hard to get to this stage. He has needed four sets to prevail in four of his six matches to reach the final and he remains a dogged presence with a big serve backed up by consistency from the back of the court.

The South African is appearing in his first major final, but Sunday's match has the feel of being all about Nadal.

Nadal has won all four of their past meetings, and while his straight sets win over him in May in Barcelona was on clay, it is hard to see how Anderson can stop Nadal if he plays anywhere near the peak of his powers, with his body now once again supporting his desire to succeed.

Even though Nadal claims that just being healthy is an achievement in itself, the icing on the cake would be a third US Open title and cementing himself at the top of the rankings most likely for the rest of the year.

Federer's return to the top of the game may have been the narrative that dominated much of the year, but it will be fitting if it is Nadal's that takes centre stage to end the 2017 grand slam season.