The Chip and Charge
Especially effective tactic against someone who doesn’t hit the corners with their serve, and who is off-balance after their serve.
What you do is stand well inside the baseline, ideally as close to the service line as you can manage while still hitting the ball, and you hit a slice “chip” back to them and rush the net. The ball will come back to them in an unpredictable way, very soon after they serve. The backspin on the ball gives you enough time to get to the net. You should generate forward momentum before your shot by taking big steps into the court before your splitstep. You should be right close to the net by the time they reach your chip.
You’re then forcing them to come up with a passing shot against you, and you have reversed the pressure onto them as the server.
The Lob and Charge
Most people have a very difficult time dealing with high topspin lobs that land deep in their court. Sometimes they’ll bounce right over the person. Sometimes they can only manage to hit a weak ball back, or lob something back at you.
One benefit of charging the net after a topspin lob is that your opponent is looking up to the ball, and they lose their peripheral vision of the net. 9/10 times they won’t even know you’ve snuck in. If they try to hit the ball normally and level it out, BAM you’re right there ready to volley it away into the short court. If they try to hit a lob back, then BAM you’re there ready for the overhead or a swinging volley. This is highly effective against people with bad footwork, stiff legs, and/or stiff arms.
Try hitting high rolling topspin shots to them and see how they react. If they always lob you back, then you know to expect a swinging volley or overhead. If they always hit a weak ball back, then close in tighter to the net and get ready for the putaway volley.
As always, for maximum effectiveness, hit your lob to their weaker side – typically their backhand.