Kei Nishikori, Agnieszka Radwanska

The U.S. Open is a money-making machine. It is the most attended annual sporting event in the world with a gold mine of corporate sponsors and a reach that dips into every continent.

But the cash flow goes beyond the USTA, which uses the money to cycle in the next crop of American tennis talent. Players have the ability to make mind-boggling salaries. The men's and women's singles champions will each make a base salary of $3.5 million. But that's just a base. It could be even more.

Since 2004, the U.S. Open Series has served as a string of hard court tune-up events for the big show in Flushing. Players accumulate points from each event. The men's and women's players with the top three point totals in the Emirates Airline US Open Series Bonus Challenge set themselves up for a boost in wealth if they win in Queens.

Kei Nishikori and Agnieszka Radwanska are the two winners this year. If either of these players win the U.S. Open he or she will receive a $1 million bonus on top of the $3.5 million base. The $4.5 million payout would be the highest in tennis history.

The double U.S. Open crown has happened five times before. Kim Clijsters did it in 2005, Roger Federer in 2007 and Rafael Nadal in 2013. Serena Williams did it twice, in 2013 and 2014, winning $4 million in total in the latter year. For two years, that total has stood as the highest single payout in tennis history.

Neither Nishikori nor Radwanska is a grand slam champion, but both have had their tastes of success. Nishikori was the runner-up at the 2014 U.S. Open and won a bronze medal earlier this month in Rio. Radwanska made the 2012 Wimbledon final and has five career grand slam semifinal appearances. Nishikori is the No. 6 seed on the men's side, and he will have to go through Andy Murray is his quarter, while Radwanska is the women's No. 4 seed and would see Venus Williams in her quarter.]MOBILE-AD-ad_dx_middle]

According to OddsShark, the U.S. Open should not stress over the extra $1 million payout. Nishikori has the eighth-best odds on the men's side at +2500 (favorite Novak Djokovic is even). On the women's side, Radwanska has the tenth best odds at +3300 (favorite Serena Williams is +120).

It won't be easy, but $4.5 million is sitting on the table if Nishikori and Radwanska can get their act together. Also, Grigor Dimitrov and Johanna Konta, US Open Series runner-ups, have a $500,000 bonus waiting if they win the tournament. That would pay them $4 million, equal to Williams' record total. Milos Raonic and Simona Halep finished third in the series and would earn a $250,000 bonus for winning the whole thing.

This is why you play the matches.