AFP, PARIS: Rafael Nadal will be crowned as the year-end world number one if he wins his first match at this week's Paris Masters, after Roger Federer withdrew from the tournament.

The two old rivals have both had stunning seasons, returning to form and fitness with two Grand Slam titles each.

But Federer, who could have still usurped Nadal at the top of the rankings with titles in the French capital and at the World Tour Finals in London, pulled out of the final event of the regular season to protect a back problem.

The 36-year-old said winning his eighth Swiss Indoors title on Sunday against Juan Martin del Potro left him unfit to play at the Bercy Arena.

“I am very sorry to have to pull out of the Paris Masters but my body needs a rest after Basel,” said the Swiss legend.

“I've played a lot of tennis this year and need to make sure I look after my physical wellbeing if I want to continue to compete at the highest level for years to come.

“I love to play at Bercy, it's a few times now that I've not played there. It's a tough one but they have to understand that it's for the cause of staying injury-free and healthy.

“I'd like to be fully fit for London and for 2018.”

Top seed Nadal, who has a bye for the first round in Paris, just needs to win his opening second-round clash to secure the top ranking heading into next year.