Rafael Nadal has revealed what his most special achievement was in a year that saw him completely turn back the clock.

Despite a sour end to his 2017 season, having had to pull out of the season-ending ATP World Tour Finals, the Spaniard can look back at the last 12 months with a sense of satisfaction.

Nadal won his first Grand Slams since 2014 with a record-breaking 10th French Open title and a third US Open title.

He also became world number one again as he pipped Roger Federer to the top spot to end the calendar year at the summit for the first time since 2013.

Reflecting on his past year, Nadal has chosen his 10th French Open title as his most special accomplishment, as he became the first player in tennis history to have double figures in a single Grand Slam.

"I am happy to feel that I am competitive again and that I have a great level," Nadal told Movistar via Tennis World USA. "My goal was, If I keep playing as I did on hard courts also on clay, I may have the chance to do something really important.

"Winning ten French Open titles is very special, something difficult to achieve. I think it's the most special achievement for me."

As for when he saw the world number one ranking within his sights, the 31-year-old states that it was during the early clay season when things started to evolve.

"During the clay swing, I was seeing how things were evolving and (Novak) Djokovic and (Andy) Murray weren't in the best form," Nadal added.

"I was second in the (ATP Race) and I told Toni: 'Well, if I play well on clay, I can be no. 1'. Then when I won Roland Garros, I really had chances."

Meanwhile, there are doubts on Nadal's fitness ahead of the 2018 season as he recently withdrew from the Abu Dhabi exhibition event which was set to take place from 28-30 December.