Roger Federer has pinpointed one particular moment in the season for losing out in the race for the world number one ranking.

The Swiss ace's long-term rival Rafael Nadal was confirmed as the year-end number one for the fourth time in his career ahead of the ongoing ATP World Tour Finals in London as Federer settled for number two in the rankings.

Federer and Nadal have had one of the best seasons in their careers having won seven and six titles respectively including equally sharing the four Grand Slams between them. It was even more impressive owing to their injury struggles in 2016, when both the players were forced to end their season early.

There was nothing to choose between the two players in the race for the top spot up until the Championships at SW19, but Federer was the favourite as the surfaces in the second-half of the season favoured his game more than Nadal's. After his triumph at Wimbledon the Swiss star raced to the final at the Montreal Masters, but was beaten in the finals by Alexander Zverev as the former picked up a back injury during the tournament.

The injury in Canada forced Federer to withdraw from the Cincinnati Masters, which handed the advantage to Nadal. The Spaniard did not fare well in Cincinnati, but was still crowned the world number one owing to Federer's absence.

The 36-year-old returned from injury at the US Open and was the favourite ahead of Nadal for the title. However a lacklustre performance in the quarter-finals saw him fall to Juan Martin del Potro. The Spaniard, on the other hand, found his best form on the hard courts when it mattered and went on to take the title at Flushing Meadows to increase his lead over Federer.

The 19-time men's singles Grand Slam champion believes Nadal's win at the US Open was when his hopes of returning to the summit in the rankings vanished. He firmly believes his long-time rival and friend deserves the number one ranking, but continues to regret that one moment that robbed him of a chance to battle for the top spot until the end of the campaign.

"All the players were playing for world No 1 but he was better throughout the year. He played more tournaments. He was incredibly successful throughout," Federer said after his win over Jack Sock in the first group stage game at the O2 Arena, as quoted by the Express.

"I don't have any regrets because I feel like in my stage of my competition, my age, either it comes to me or it doesn't.

"He had more gas left in the tank than I did. I couldn't play as much as I've wanted, or I overplayed anyway. My only regret was I would have loved to be in contention through the Montreal final, Cincinnati and US Open," he explained.

Roger Federer
Roger Federer is currently playing the ATP World Tour Finals in London Getty

"But things evaporated very quickly after he won the US Open. That's when he made his final push, in Asia. That was it for me really.

"In some ways I'm happy he clinched it because he deserves it. And like this, I can focus on playing the tournament [the ongoing ATP Finals], and not having to talk about that at the same time.

"But, sure, I would have loved to be No 1, but you can't be it with that many tournaments, so it's no problem for me," he added.