Just because you lack a playing partner from time to time, that doesn't mean you cannot work on your tennis game.
Check out some tips on "Wall Ball" from Nick Bollettieri to Tennis.com (click the link for the whole article):
1. I like to tell my students that there's only one undefeated champion in the history of tennis: the wall. It never misses, never complains and never loses. To get the most out of it, though, you need the right approach. First, make sure your wall has a line to represent the net. Measure this and if you can't draw it perfectly, make it a little higher, rather than lower. When you start your wall practice, stand far back and hit high, slow-to-medium speed shots (keep in mind that the distance from the baseline to the net is 39 feet). Give yourself time to take back your racquet and get into a rhythm. Since the wall never misses, you don't want to start out at top speed and take a lot of hard swings when your body is cold.