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The victory follows on the heels of Zverev winning the men's singles gold medal at the Tokyo Olympics.
The field still boasts world No. 1 and reigning Wimbledon champion Ash Barty, four-time Grand Slam winner Naomi Osaka and reigning French Open champion Barbora Krejcikova.
Two-time Grand Slam winner Azarenka, who won the Cincinnati Open in 2013, will return to the women's draw for the first time since 2016.
The former world No. 1 says he is 'closer than he thought' to being ready.
Rublev needed just over an hour to beat Federer 6-3, 6-4 in the third round of the Cincinnati Masters, the Swiss 20-time Grand Slam champion suffering his quickest defeat for 16 years.
Osaka was trailing American Sofia Kenin in the third set of their quarterfinal in Cincinnati when she sought treatment on her heavily-strapped left knee. After briefly returning to play, the Japanese star retired with Kenin leading 6-4, 1-6, 2-0.
It's the first time, Nadal has retained a title outside of clay.
Medvedev seized his maiden Masters shield in Cincinnati, extending a dominant run that saw him reach three finals in three weeks.
The Scot lost 6-4, 6-4 to Richard Gasquet in his first singles match since January at the Cincinnati Masters before saying he had turned down a wild card into the men's draw at the year's final Grand Slam.
Results 1-9
