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TORONTO — The afternoon before the biggest match of his career on home soil, Gabriel Diallo had an hour-long practice that more than 50 fans watched, many of them holding giant tennis balls and hats for him to sign, others recording his monster serve on cellphones they later used for selfies with Diallo, who joined them with a smile. 

“You can beat him tomorrow!” one fan yelled, referring to Diallo’s upcoming third-round tilt against American Taylor Fritz, the world No. 4. Diallo glanced...