The Australian has stormed to the No1 spot after winning the BMW Championship by six shots in Illinois this weekend.
For what was his fourth tournament win since July and fifth of the season, Day picked up $1.49m in prize money to take his total for 2015 to $9.2m. In the last two decades, only Tiger Woods and Vijay Singh have won as many titles in a single season.
Day's triumph at the BMW Championship follows three wins at his previous five PGA Tour events since the Open - a run which included his first major title at the US PGA Championship and biggest ever payday of $1.8m.
In his last six events he has scored a staggering 101 under par.
Two months ago the 27-year-old had made $3.6m from his performances on the course but such remarkable run of form in recent weeks has seen him almost triple his earnings for the year.
In the last eight weeks Day has made:
- Over £1m more than the Premier League's highest-paid player Wayne Rooney (who will have made roughly £2m in the period).
- Roughly the same amount that Andy Murray has made in the entirety of 2015.
- Around half of what highest-paid FTSE 100 chief Sir Martin Sorrell made.
- More than twice as much as rugby's highest paid player Dan Carter makes in a year.
Only Jordan Spieth stands ahead of Day on the 2015 PGA Tour Money leaderboard having made over $10m from a breakthrough season in which the Texan became the first player to win the Masters and US Open back-to-back since Tiger Woods in 2002.
However, Day is well placed to finish the season as the most well-remunerated player with the FedEx Cup's $10m end of season bonus still up for grabs.
A win at next week's end-of-season Tour Championship at East Lake offers him the chance to add an extra $10m to his bank account.
The winner's paycheque from the Tour Championship only stands at $1.5m but the tournament will settle the result of the FedEx Cup, the trophy awarded to the PGA Tour's season champion.
Although the FedEx Cup's $10m paycheque technically counts as a "bonus" rather than prize money, it is the biggest financial reward available in golf.
And Day is currently best-placed to win the prize as he sits atop the standings on 6,680 FedEx Cup points to Spieth's 4,392 in second place.
A sixth tournament win of the season would guarantee Day or any of the top five (Spieth, Rickie Fowler, Henrik Stenson and Bubba Watson) the lucrative prize, but it is even possible for him to finish as low as 29th and still finish top of the pile if results go his way.
|Rank||Player||2015 FedEx Cup points||2015 PGA Tour titles|