Roger Federer's defeat to Juan Martin del Potro means the Spaniard will keep top spot in the ATP rankings
Rafael Nadal guaranteed to stay world No 1, but winning the US Open will prove he is worthy of it
Rafael Nadal had already won once on Wednesday in New York when his second victory of the day came a few hours later.
The world No 1 had already booked his spot in the semi-finals of the US Open with a straight sets win over Andrey Rublev.
But, Roger Federer's surprise defeat to Juan Martin del Potro in his quarter-final clash guarantees that the Spaniard will stay top of the world rankings come the conclusion of the tournament on Sunday, even if he does not go on to win for a third time.
Nadal's status as No 1, which he had earned last month had been shaky, with both Federer and Andy Murray in with a chance of taking it from him ahead of the last major of the season.
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But Murray withdrew with the hip injury that had kept him out of action since Wimbledon, while Federer, who would have been guaranteed No 1 if he had won a sixth title at Flushing Meadows, fell to Del Potro in four sets.
The question now is can Nadal, 31, do what Federer could not and beat a confident Del Potro and go on and win the title to underline his status as the best player in the world?
That is not to say that Nadal is not worthy already of the top spot. He reached the Australian Open final, had an excellent clay-court season that culminated in winning a 10th French Open crown and first in three years.
His recovery to the top of the game was almost overshadowed by the exploits of Federer, who beat him in Melbourne, won Wimbledon, and has three other titles to his name.
It is arguable that if Federer had chosen not to miss the clay-court season to allow him to prepare for his tilt at an eighth Wimbledon crown that he would be world No 1 now, given the form he was in at the time.
But that was his choice. He chose not to play. He was not injured, and unlike him, Nadal has stuck to a full schedule and not been tactical about where and when he plays.
Friday's showdown with Del Potro will be his 66th match of the season, already 11 more than he played in an injury-hit 2016, and only 15 less then he played in 2015.
While all of his four titles this year have come on clay, Nadal has returned to being a force in the game after struggling to match Novak Djokovic, Murray, Federer and even Stan Wawrinka in recent years.
He was runner-up in Melbourne, a beaten finalist at the Mexican Open, and lost to Federer again in the Miami Open final.
Having fully recovered from a wrist injury that wrecked his French Open chances in 2016 and forced him to end the season early in October to recover, he has shown no signs of ill-effects and has proven more then a match fore the majority of his younger rivals.
Winning the US Open to add to his 2010 and 2013 victories would be the icing on the cake of an excellent year. It would also get rid of the misconception that his year has been built purely on his excellence on clay.
Yes, it is the high point right now, undeniably, but Nadal has been consistent on hard courts, and had his best run at Wimbledon in three years.
It took a supreme effort from Gilles Muller to put him out in the fourth round in a match that lasted almost five hours, a marked improvement on his meek past efforts at SW19.
If Nadal loses to Del Potro he is still set to have a ranking points lead of more than 600 points over Federer when the new rankings are published on Monday.
Both have few points to defend in the final three months of the year, with Federer's 2016 season ending in July, and Nadal only has 110 points from the two tournaments he played after New York last year. A fascinating tussle for the world No 1 spot could be on the horizon.
However, if Nadal wins his 16th major on Sunday then he will be almost 2,000 points ahead of Federer and that will pretty much guarantee that when he starts his preparations for the 2018 season - beginning at the Mubadala World Tennis Championship in Abu Dhabi in December - as world No 1.
But to do that he must first get past Del Potro, with either Kevin Anderson or Pablo Carreno Busta waiting in the final.
Del Potro is in sensational form. To come from two sets down to beat Dominic Thiem in the fourth round was impressive, but to make Federer look so ordinary on Wednesday was a terrific feat.
Nadal, who dropped only five games in beating unseeded Rublev in his quarter-final, will be the fresher man, and has a 8-5 record over the Argentine.
But, Del Potro will have big crowd support behind him, plus he won their only meeting in the past four years at the Rio Olympics in three sets.
While a lot was made of Del Potro's victory over Federer, repeating his 2009 success over the Swiss for his only grand slam title to date, he also beat Nadal in the semi-finals, a feat the Spaniard will hope doesn't repeat itself when they next meet.
Nadal has had a fine season, but he now has the opportunity to make it a truly great one if he can win two majors in a year for the first time since 2013.