Kei Nishikori is recognized as the best Japanese player. Recently, Wilson Racquet Sports decided to continue his multi-year contract probably because he is an important part of the overall marketing strategy for Wilson. He has been a part with Wilson for years and it started when he was only ten years old. Since Nishikori has a strong following in Japan, it only makes sense that he is an important part of the brand over there.
There are other possible reasons too. Not only is he on the top of the list in Japan, but he also has returned to tour after recovering from a elbow surgery and has made an impressive comeback. Some of his latest achievements include winning titles at the ATP Challenger Tour in the Savannah Challenger. Then a win at the Sarasota Open. In the US Open, he ousted the 11th seed Marin Cilic to move into the third round. His plans are to play the entire fall schedule in Europe and Asia.