An elbow injury knocked defending champion Novak Djokovic out of this week's Miami Open.

Despite his absence, Djokovic earned another title this week: Toughest opponent Kei Nishikori has faced.

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Nishikori stunned Djokovic in the 2014 US Open semifinals to reach his lone Grand Slam final.

Since that match, Djokovic has reeled off nine consecutive wins over the Japenese, including straight-sets wins in the 2016 Miami and Rogers Cup Masters finals.

“Maybe the record show maybe Djokovic,” Nishikori said when asked which of the Big Four is his toughest opponent. “I haven't beat him long time. But everyone is tough, you know, with these days. There is many young players coming up and there is no easy matches.”

Asked to assess the Big Four, Nishikori said Roger Federer is the most aggressive and Andy Murray owns some of the best groundstrokes. Nishikori said Federer plays “a little bit opposite” Murray.

Federer is 5-2 lifetime vs. Nishikori, while Murray has won eight of 10 meetings with the Japanese.

“I think Andy is I think one of the toughest groundstrokes players I think. He doesn't give you many errors,” Nishikori said. “He doesn't give you easy points. He has great serve and one of the best return. He gets many balls. So one of the toughest player for sure.

Yeah, it's something different than Rafa or maybe Roger. Roger is extremely aggressive and maybe a little bit opposite than Andy.”

Photo credit: Mark Peterson/Corleve