Novak Djokovic believes there is "room for improvement" regarding the prize money available to players at the four Grand Slams.
Tournament prize money has been a contentious issue this year, after the top 20 men's and women's players signed a letter asking for a greater share of revenue from the Australian Open, French Open, Wimbledon and US Open.
Earlier this month, the United States Tennis Association announced a 20% increase in prize money for the US Open - creating what is claims is "the largest purse in tennis history' - with $90m (£67.