Kei Nishikori is the only male Japanese tennis player ever to be ranked inside the top 5 in singles, and first reached his career-high singles ranking of world No. 4 in March 2015. He is currently ranked world No. 12.
He often jumps before shots a ball. He also twists his abdominals, so he can use muscle reflex and swing his arm fast.
He had experience he strained his abdominals a couple of years ago. That means he uses his abdominals a lot.