At best, it means that the knee still doesn't feel very good and he wants to get the maximum amount of hitting in without undue stress on the knee.
At worst, the knee doesn't feel good at all and he is just mailing in the spring hardcourts to try and get maximum time before he has to defend his clay points.
Knock knock. Who's there? Knock knock. Who's there? Knock knock. Who's there? Philip Glass.