Roger Federer will not participate in the Tokyo Olympics, writing on social media that he "experienced a setback" with his knee during the grass-court season.

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setback セｯﾄ ﾊﾞｯｸ n)つまづき･挫折･後退

knee

grass-court　

experience a setback 進歩などの後退を経験する

a big/minor/huge/major/financial set back