In this July 7, 2021 photo, Switzerland's Roger Federer returns against Poland's Hubert Hurkacz during their men's quarter-finals match on the ninth day of the 2021 Wimbledon Championships at The All England Tennis Club in Wimbledon, southwest London. (GLYN KIRK / AFP)

Roger Federer was the highest earning tennis player in the world over the last 12 months, making about US$90.6 million before taxes, despite the Swiss maestro's time on court being limited by injury, Forbes' said on Thursday.

The 10 highest-paid tennis players collectively earned US$320 million over the last 12 months

Federer, who has skipped this year's US Open due to a knee injury, made less than US$1 million from playing tennis, with the bulk of his income coming from endorsements, Forbes' said.

Japan's Naomi Osaka is the second-highest earner on the list, with US$60.1 million in pre-tax income, more than 90 percent of which she made off court, with Serena Williams, Novak Djokovic and Rafa Nadal completing the top five.

Williams and Nadal are also out of the year's final Grand Slam due to injury.

The 10 highest-paid tennis players collectively earned US$320 million over the last 12 months, down 6 percent from last year, largely due to tournaments and lucrative exhibition matches being cancelled due to the COVID-19 pandemic, Forbes' said.

Despite the decrease, the top 10 made more money off court, taking in US$281 million from endorsements, or 4 percent more than the previous year, even with reduced spending by a number of brands.

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Top 10 highest earners in tennis:

Roger Federer - US$90.6 million

Naomi Osaka - US$60.1 million

Serena Williams - US$41.8 million

Novak Djokovic - US$38 million

Rafa Nadal - US$27 million

Kei Nishikori - US$26 million

Daniil Medvedev - US$13.9 million

Dominic Thiem - US$8.9 million

Stefanos Tsitsipas - US$8.1 million

Ash Barty - US$5.8 million