The Arthur Ashe Stadium can be an intimidating arena. It is the biggest tennis stadium in the world and with a rapturous New York crowd, it can unsettle the best in the business. In the 2009 US Open final, Juan Martin del Potro, all of 20, faced a daunting task – he was up against Roger Federer, a five-time defending champion and an overwhelming fan favourite. Yet, the Argentine held his nerve and upped his game in the most remarkable fashion to script one of the greatest upsets in sporting history.

Federer came into the tournament having enjoyed a historic season up until then. He had finally got his hands on the French Open trophy and had followed that up by reclaiming his Wimbledon crown and the No 1 ranking by defeating Andy Roddick in an epic at the All England Club.

The Swiss had a smooth run to the final. In his six matches, he only dropped two sets – one in the third round against Lleyton Hewitt and another in the quarter-final against Robin Soderling, the man he defeated at the Roland Garros final earlier in the year. In the semis, Federer put in a dominating performance to beat fourth seed Novak Djokovic in straight sets.

Del Potro, on the other hand, was also having a phenomenal tournament. Just like Federer, he had dropped just two sets in the run-up to the final and in the semis, he registered a stunning 6-2, 6-2, 6-2 win over third seed Rafael Nadal.

Also read: How the Grand Slam finals in 2009 transformed Federer, Nadal’s tennis legacy

The final had all the makings of a blockbuster and oh boy, did it deliver. Federer was the firm favourite, leading 6-0 in the head-to-head record which included victories at the Australian and French Opens earlier in 2009, but Del Potro’s performances up until then were enough for fans to know that he had the firepower to take down the best.

Federer started strong, dominating the opening set to take it 6-3. He kept up the momentum even as his opponent started to find his range and at 5-4, he was serving for a two sets to love lead. Had he closed it out, the odds of him winning the match would’ve been extremely high. After all, at that point in his career, he had a win-loss record of 149-0 at Grand Slams when up two sets to love.

Del Potro, however, made his first big move in the match and broke back to take the second set to a tie-breaker. He then showed incredible composure to win the set and level the match.

Federer, though, bounced back and despite being a break down at 3-4 in the third, he broke the Argentine twice in a row to get his nose ahead once again.

The fourth set was another rollercoaster as both players traded breaks of serve to take it to a tie-break. But there was one critical change that had happened by then – Del Potro had found his top gear and was hitting the ball as clean as a whistle. He took the tie-break again and the match headed into a final set shootout.

While the majority of the fans in the stadium and millions watching from home wished for another Federer fightback, Del Potro was going to have none of it. He launched a sensational assault on the Swiss, blasting forehands with a kind of power that had never been seen before on Ashe. It was a sight to behold, Federer would have the ball back under his nose almost as he completed his previous stroke. His opponent had well and truly entered beast mode.

Del Potro broke Federer’s serve twice to take the fifth set comfortably and win the match 3-6, 7-6(5), 4-6, 7-6(4), 6–2. It was a jaw-dropping performance from the 20-year-old in his very first Major final. And even the crowd in the iconic stadium, which may have been heartbroken at the time, couldn’t help but stand and applaud what the youngster had achieved.

For Federer, in hindsight, it was a massive defeat. He had won the French Open and Wimbledon that year and would go on to win the Australian Open in 2010. Had he won that one set more against Del Potro in the 2009 US Open final, he could’ve held all four Majors at the same time. This was the closest he ever came to achieving that feat.

“At the time I could not explain what had happened when I won the US Open and now, all these years on, the feelings are the same,” Del Porto told Tennis365 in 2018. “I beat Nadal in straight sets and played an unbelievable match in the semi-final, but then you have to beat Federer in the final to achieve your dream and it is a very difficult challenge.

“I remember the last set and thinking I just have to win. I might never get a chance like this again. I have to win and many times you think it will not happen. Then the moment you win and the dream is here. It is all over. You win and forever more you have won the US Open. It is the most amazing feeling.”

Watch the entire 2009 US Open men’s singles final, highlights of it, and interviews of both players after the match: