Plyometrics are NOT a cardio activity, in fact done correctly (and if you're fit) you'll barely break a sweat. I'm not saying they're bad, because they will in fact improve your performance on the court as flippinfreak suggests, they're just not an adequate replacement for your tennis CARDIO requirements.
I would go for some nice long runs, maybe building up to 45 min runs for endurance, interspersed with cardio-vascular threshold training sessions (if you can hit 5km in 20 mins or less you're doing quite well) Of course, running can be quite boring so you may want to mix it up a bit with some swimming and cycling as well, anything to get that heart-rate right up. Basically you want to increase your cardio-vascular endurance (ie fitness) for the next 8-12 weeks, then start throwing in some sprint and agility work (shuttle sprints etc) to increase your lactate threshold (ie your 'burn' tolerance) and improve speed and agility over short distances as is necessary in tennis. This is also the period in which plyometrics should be done as well... the reasoning? You will have built up a bit of a strength base which is ideal preparation, plus it is a natural partner to sprint-type exercises. By the way, don't neglect the cardio-vascular stuff during this period, just scale it back a bit. This will last probably 6-8 weeks which should transition nicely into pre-season which is where skills work should become the primary focus.
It's called 'training periodisation'...
