Andre Agassi calls Rafael Nadal a freak of nature, Pete Sampras says he is one of the strongest men ever to walk onto a tennis court. Yet if the Spaniard emulates the feat of his great rival Roger Federer and becomes the second player this year to complete a full collection of the game’s major titles it will rate as the supreme accomplishment of his stunning career.

Physically, Nadal has no right being in today’s raindelayed US Open semi-final. After a summer spent wondering how his troublesome knees would react to the rigours of seven best-of-five set matches on a surface that jolts and jars his sinews, he has also had to battle against an abdominal strain that makes each serving motion agony.

Common knowledge already decreed that the 23-year-old’s resolve was a thing of wonder. Basic common sense insists that, even before the weather intervenes, the concluding programme of the year’s last Grand Slam event is uneven and inequitable.

But should Nadal, a 7-6 7-6 6-0 winner over Fernando Gonzalez in a quarter-final that began before the squall set in almost two days earlier, prevail by registering three victories in as many days then the world of tennis will acclaim him as a racket-wielding gladiator with no peer.

Today will hopefully decree whether a Federer v Nadal final will grace the Arthur Ashe stadium, as it has Wimbledon’s Centre Court, Roland Garros’s Stade Philippe Chatrier and Melbourne’s Rod Laver Arena. To date, they have played 20 times, with the total standing 13-7 in the Spaniard’s favour.

Before Nadal can think about Federer, he must first overcome Juan Martin del Potro, the towering 20- year-old Argentine who has usurped Andy Murray to take the unofficial mantle of “the next major champion in waiting”.

Nadal knows he faces a tough task. “Now it is Juan Martin and he is a very good player,” he said. The Spaniard holds a 4-2 lead over Del Potro but lost their past two meetings, both this year. “Right now he is very solid from the baseline and serving unbelievable so I must play my best match to beat him." As the outer extremities of Tropical Storm Fred deposited continuous rain on Flushing Meadows, Del Potro had all of Friday and then yesterday to rest and recuperate while his upcoming opponent grew ever more impatient and disadvantaged.

Del Potro succeeded where Murray failed on the cement of the Arthur Ashe stadium. It does not matter that the Scot has got the better of him in four of the five matches in which they have played one another, including the final of Montreal’s ATP World Tour Masters 1000 event less than a month ago and last year’s US Open quarter-final. Del Potro, who is 6ft 6in, succeeded in overpowering the equally statuesque Croatian Marin Cilic whereas Murray, so hopeful of contesting a second major final after last year’s confrontation with Federer, felt flat, outclassed and uncompetitive against the same opponent.

How quickly opinions change. Now it is Del Potro who is perceived as the young man most likely to succeed. The shy giant from Tandil, a small city in Argentina with a proud tennis heritage, is deemed to have the weapons necessary to go all the way. His height, weight of serve and forceful forehand are thought to be crucial in Grand Slam tennis and he is starting to believe in his ability to battle with the game’s elite.

Crucially, Del Potro appointed a new coach 18 months ago, Franco Davin, a fellow Argentine who still holds the record for being the youngest player to win a tour match after beating Chile’s current Davis Cup captain Hans Gildemeister aged 15 years and one month. Davin coached Gaston Gaudio to the French Open title in 2004 and came straight to the point with his new charge. “He told me you have to wait for the moment and be patient,” recalls Del Potro. “But you have to be good when the opportunity comes and that means you must work hard.”

Davin believed Del Potro’s lack of conditioning accounted for his many injuries and an erratic serve that could deliver bombs one moment and sitting ducks the next. Laid-back to the extent that he can quite easily sleep all day, the player needed some forcing but he knew the work had a purpose. “I knew if I was going to even think about beating Nadal or Federer I needed to be stronger and more aggressive but I needed more time,” he said.

Progress has been apparent this year, with a quarter-final at the Australian Open and a semi at the French where he lost to eventual champion Federer over five brutal sets. However, an ill-timed double fault in the fifth ultimately proved costly and it was time again to focus on his serve.

“Sometimes he would serve unbelievably and other times he would just put the ball in court,” said Davin. “When he gets tired he relaxes and the first thing that goes is his serve. That needed to change.” So it did. Del Potro duly lost at Wimbledon, but American cement is a different matter and, unlike many of his countrymen, he views the US Open, rather than the French, as the most cherished tennis prize. Since leaving Wimbledon, he has amassed an impressive record of 16 wins and just one defeat; that match against Murray.

Del Potro is revelling in the moment and mindful that the elements are lending him a hand. No South American has won this title since Guillermo Vilas in 1977 when it was played on green clay at nearby Forest Hills. Many educated observers feel the wait could soon come to an end and, most importantly, Del Potro finally believes the gap is narrowing between him and the likes of Federer and Nadal as well as Andy Murray and Novak Djokovic.

In the juniors at Port Washington, Laura Robson won her quarter-final against unseeded American Lauren Davis 6-2 6-3 while Heather Watson beat world junior champion Noppawan Lertcheewakarn of Thailand 6-2 6-1.

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