When you do not have a training partner but still need to put in the work to keep in shape or develop your tennis skills, you have to get a little bit creative with your drills. The fitness elements are more straightforward to complete as an individual but the on-court drills are more difficult to do alone. If you have the luxury of a court and all the equipment you could need then solo drills are easy but not many people do have all the expensive gear.
However, there are plenty of drills that you can incorporate into individual training that will help you to progress with your tennis training, such as:
Whilst returning volleys from a wall will not replicate the exact ball direction that you might receive from an opponent, it is still a great way of sharpening up your reflexes and speeding up your reaction time. So hit a ball against the wall and repeatedly return it. You can practice doing it whilst allowing the ball to bounce or practice a direct volley from the wall. As well as helping with your racquet work, this is also really good for improving your footwork/foot speed.
If you have a free court to practice on but just no tennis partner then you can still spend time perfecting your serve. You can either bring a large volume of balls with you, or you can incorporate ball retrieval into your fitness training. So for example, after hitting 10 serves run round to the other side to collect them all, making sure you are doing sprints to each ball in the way that you would move around the court during a game. Then serve them to the other side and so on. If you do not have a court, you can still mark out an area on grass or concrete to fire your serves at. Ideally you would set your area up somewhere near a wall or fence to keep the balls nearby.
No racquet required for this drill, as you are working on your speed and endurance. If you are able to use a tennis court then you have your line markings ready to go. You run from the sideline to the intersection line, touch the line and turn and run back to the sideline. The next run is to the other sideline and back, then you repeat these shuttle runs. It is a similar drill to the bleep test and you can keep doing this for as long as possible, trying to improve the number of shuttles each time you do this one.
A good way of improving your footwork is to use ladders. In the old days people actually used wooden ladders that they lay on the floor for people to do this drill but now you can buy ladders made from lightweight materials. So you lay the ladder along the floor and step into the first square with one foot and then the other. You do the same for each square of the ladder until you get to the other side. For tennis, it is helpful to try doing this forwards, backwards and sideways. You can run back round the edge of the ladder and do it again, or incorporate it into a circuit drill with a number of other drills.
Depending on your fitness and jumping ability, you can possibly use the net to perform sideways jumps from one side of the net to the other. If you are not quite at that jumping level then you can practice using a smaller item but being able to jump is important in tennis and this drill can give you a good workout for developing your leg muscles. You can also do it using a small wall or other surface to jump up onto the top off and then back down and repeating. To make it replicate a match situation more closely, try and jump high on the spot hitting an imaginary ball rather than using anything to jump over or onto. This will help you to reach those high balls in match situations.