Future uncertain for International Premier Tennis League.
The first international blind tennis tournament was held in 2017.
Stars gear up for the holiday season pumping up.
Former No. 1 tabs Zverev as future No. 1 and offers alternate view on champions coaching.
The former U.S. Open runner-up owns 35 titles between singles and doubles.
World No. 2 hasn't officially retired from Swiss Davis Cup squad.
The Spaniard will take over for Manolo Santana in 2019.
