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MADRID, SPAIN - MAY 09: Stan Wawrinka of Switzerland returns the ball in his match against Kei Nishikori of Japan during day six of the Mutua Madrid Open at La Caja Magica on May 09, 2019 in Madrid, Spain. (Photo by Alex Pantling/Getty Images)

MADRID, SPAIN - MAY 09: Stan Wawrinka of Switzerland returns the ball in his match against Kei Nishikori of Japan during day six of the Mutua Madrid Open at La Caja Magica on May 09, 2019 in Madrid, Spain. (Photo by Alex Pantling/Getty Images)