Novak Djokovic and Roger Federer will continue their long-standing rivalry on Sunday when they square off in the final of the ATP Tour Finals.
The pair had already met earlier in the year's event, when Federer won 7-5, 6-2 to end Djokovic’s 23-match winning streak this season. That victory means Federer leads their head-to-head match-up 22-21.
Sunday's match will be the eighth time they have met in 2015, with Djokovic leading four wins to three.
Djokovic has only lost 10 matches the whole year, but Federer his the only player to have beaten him more than once, doing so on three occasions.
The Serb triumphed in Rome, Indian Wells, Wimbledon and the US Open, while Federer won in Dubai and Cincinnati prior to his victory in London this week.
Between them, the pair has won 10 of the last 12 season-enders, with Federer winning six and Djokovic four, including the last three. Sunday's match is also a rematch of the 2014 final, which was awarded to Djokovic after Federer withdrew from the match with a back injury. They also met in the 2012 final, which Djokovic won in straight sets.
Should Djokovic win the 2015 title as well, he will become the first player to claim four straight titles in the tournament's history.
Djokovic has already been confirmed as the world number one for 2015 after winning three of the four grand slam titles of the year as well as Masters Series titles at Indian Wells, Miami, Monte Carlo, Rome, Shanghai and Paris, but the number two spot is up for grabs.
Should Federer win on Sunday he will move up to second on the rankings, overtaking Andy Murray, but the Scot will have another chance to finish the year as the world's next best player for the first time in his career if he wins both his singles rubbers in next week's Davis Cup final.
"It's the best season of my life," Djokovic said after his victory over Rafael Nadal in the semi-final.
"Regardless of the outcome of Sunday's match, the season will still be the best that I had.
"I don't feel unbeatable, but it does definitely feel great when I'm playing the way I played.
"When I feel like I'm dictating the pace, the match itself, most of the rallies, I feel like I'm on top of my opponent."
Djokovic is the undoubted best player in the world currently, but Federer is seemingly the only player on the circuit capable of upsetting the record-breaking Serb.
Whilst Djokovic has triumphed in the big matches more often than not, Federer has had the gameplan to beat him.
"For me it's an advantage because I guess it gives me a bit of confidence," Federer said of triumph over the world number one earlier this week.
"For him it's an advantage because he gets a second chance, and he's in another final. He's played some great tennis since we've played.
"So I still believe his confidence is slightly higher than mine with the amount of success he's had this year."
Sunday's final at London's O2 arena certainly has all the ingredients of being another classic, considering the level of tennis often played when these two greats square off.
Djokovic starts as the undoubted favourite, but if anyone can beat him to the title this time around, it is the 17-time grand slam winner.