With his title at the ATP World Tour Finals, Novak Djokovic is guaranteed to become the first player in tennis history to earn $20 million in a single season.
Djokovic's on-court excellence has been well-rewarded in 2015. His victory against Roger Federer in Sunday's year-end championships triggered a payment of $2.06 million, which brought his prize money total to $18.77 million.
Djokovic's figure for this season will be bumped up by his end-of-year bonus from the ATP, with the Serbian to receive an additional payment of $2.825 million for finishing as the world No. 1 and playing in seven of the eight Masters-level tournaments.
That will give Djokovic a grand total of $21,595,000 this season -- before endorsement and appearance fees.
Only nine players have earned more money in their careers than Djokovic will earn this season, according to ESPN Stats & Information.
Djokovic's dominance this year thrusts him into a race with Federer to become the first player to break the $100 million mark in career earnings, which excludes bonuses from the ATP.
Djokovic's career tally for prize money stands at $91.22 million, which isn't far behind the Swiss star's. Federer's run to the final was worth $1.12 million, which brought his total for the year to $7.81 million and his career figure to $96.43 million.
Barring injury or a precipitous collapse in form, at some point in the 2016 season, those two will become the first players to earn nine figures. It's almost certain the "Big Four" will soon occupy the top four positions in all-time earnings.
Rafael Nadal's run to the semifinals in London, after going undefeated in his group, was worth $668,000, which brought his total for the year to $3.94 million and his career figure to $75.32 million, the third-highest of all time.
Pete Sampras holds fourth position with $43.28 million, but Andy Murray is moving ever closer to that figure. Although Murray didn't make it into the knockout rounds in London, he still earned $334,000 from the season finale, which lifted his total to $6.82 million in 2015 and $41 million in his career.
This is the fifth successive season in which Djokovic has earned in excess of $12 million.