Roger Federer holds the record for the most number of Grand Slam titles and number of weeks as the No.1 but has a poor head-to-head record against his longtime rival Rafael Nadal.
Rafael Nadal has withdrawn from the ATP World Tour Finals after his knee injury flared up during his loss to David Goffin on Monday (13 November).
Roger Federer was favourite to become world No1 going into the second-half of the campaign until he suffered a back injury during the Montreal Masters.
Roger Federer does not agree with Rafael Nadal's viewpoint that the ATP Finals in London should be played on clay.
Roger Federer revealed that he feels fully fit after coming through unscathed in his first group stage win over Jack Sock at the ATP Finals in London on Sunday, 12 November.
Marin Cilic has given his assessment on whether Rafael Nadal and Roger Federer benefited from the 2017 absences of Novak Djokovic and Andy Murray.
Agassi says Nadal's end of the year performance is "never quite the same as the middle or the beginning".
World number three Alexander Zverev believes 2017 was the year Rafael Nadal and Roger Federer played the best tennis of their careers ahead of the ATP Finals in London.
Goffin has met Nadal on two occasions in 2017, both on clay in Spain and Monte Carlo.
Rafael Nadal remains a doubt for the ATP World Tour Finals after admitting he is yet to decide whether to compete at the season-ending tour finals.
Hip and elbow injuries kept both players out of top form for a major part of the year.
ATP are trialing a number of new rules and innovations at the Next Gen ATP Finals in Milan with a view to introduce it on the regular tour in the future.
Djokovic started the year as the number two ranked player in the world but he has currently fallen to number seven after what has been a poor, injury-ridden year.
The 36-year-old has won seven titles this year, including the Australian Open and a record eighth Wimbledon title to overtake Sampras' seven championship wins.
Nadal has faced Federer in the finals of the Australian Open, the Miami Open and the Shanghai Masters, as well as in the fourth round at Indian Wells.
Roger Federer skipped the entire clay court swing of the season in 2017 in order to remain fit for the grass and hard court events later in the year.
Toni Nadal will step down from Rafael Nadal's coaching team when the season concludes after the ATP Finals in London next week.
Rafael Nadal and Roger Federer's extraordinary year was unpredictable but it is also proof that great athletes shouldn't be questioned according to the latter's ex-coach.
Nadal is keen to win the season-ending ATP Finals clash for the first time in his career but is aware that he could come up against Federer in the finals.
Rafael Nadal is confident he will be fit in time to play in the ATP World Tour Finals, despite a recurrence of his long-standing knee issues.
Former world number one Mats Wilander believes Rafael Nadal is the only player 'hungrier' than Roger Federer as he details the changes the Spaniard has made in 2017.
Rafael Nadal has been given a clean bill of health going into the ATP Finals by coach Carlos Moya, who has backed him to challenge for the win.