That could be a wild guess.
As a hitting partner, he need only to hit about the pace and spin of the WTA girls, NOT like a 5.5 man, serve moderately, like to stay attached to the baseline, be a good travelling companion, be ready to hit whenever needed, and to hit, not play, during the hitting sessions.
MichaelChang's regular hitting partner was his brother Carl, a former Div1 player at the medium level. Mike was a 7.0 MAN.
I was a regular hitting partner for two years for both Mareen and Marcie Louie. Then wanted to face a lefty serve mostly, but our sessions were the full one hour allowed at GoldenGatePark. I was not to volley, serve at the body, or drop shot them when we played practice sets. I was a winning C player then, C is 3.5.