He also was Djokovic's warm up for his semi with Fed. Jack told a funny story that he got the call from Djok and then 30 min. later he got a call from Roger asking him to practice with him! He had to turn Fed down because he had already committed to Djok. How cool for Jack to have such superstars asking for his assistance.
Wow that's great and funny also
The U.S. needs to find young talented tennis players.