Novak Djokovic and Roger Federer are set to become the first tennis players in the history of the game to reach $100 million in on-court earnings next season.
World number three Federer has banked $97.3 million during his career while Djokovic – six years the Swiss ace's junior – has $94 million following a stellar 2015 season.
With a first prize of $3.85 million on offer at next months Australian Open, Federer could became the first player to reach the magical number as early as January, even though the odds are heavily stacked in favour of the Serbian world number one.
Boosted by lucrative sponsorships and endorsements, the earnings of modern players dwarfs those of their predecessors.
The last man to win the calendar year Grand Slam – Rod Laver – retired with career earnings of $1.5 million, which was huge sum in the 1960s.
John McEnroe, meanwhile, earned $12.5 million and Pete Sampras had banked $43 million by the time he retired in 2002.
Djokovic earned a staggering $21.5 million in 2015 having won three grand slams, lost in the final of the other one and claimed seven other titles in what proved to be a stellar year for the 28-year-old.
"My season was the best of my career with many highlights. It inspires me even more to keep on going, and I hope to continue to play at this level in 2016," said Djokovic.
"I think I have a good chance. I'm 28 and I still don't feel like the end is anytime soon, that definitely excites me and motivates me to keep going."
Djokovic could soon eclipse Federer's on-court earnings, but the 34-year-old 17-time grand slam winner's overall personal fortune dwarfs that of the world number one.
Federer was the fifth highest-earning sportsman on the Forbes' rich list in 2015 thanks to $58 million in endorsements, while Djokovic was 13th with $31 million.
The level of earnings in the men's game far eclipses the women's. World number one Serena Williams had a similarly dominant year on the WTA Tour, but her on-court earnings stand at just over $74 million.
Meanwhile, Maria Sharapova, the world's richest sportswoman due to her lucrative off-court portfolio, earned $36.4 million during 2015.