Depends what you means by heavy workout. Well, running left and right and hitting the ball is completely different than lifting weight with proper form for reps. I 'll take it that you have never worked out.
It's obvious to me they don't workout during tournament, and have slight workout during weeks-off.
actually ive been working out for the past 4 years and im a qualified physiotherapist who plays tennis himself.
and no theyre not 'completely different' at all, considering both of them involve using your muscles to create power. I do strength training in the gym, which I imagine is what they do as well, low reps (5-8 per set). I can play a tennis match the next day with minimal pain. So even though they would be playing matches at a much higher level, their bodies are used to be strained and so they can recover quicker.
Also if you check Marcos Baghdatis' twitter account, you'll see that he tweets that he goes to the gym during tournaments, so the fact you think its 'obvioous' they dont work out during a tournament is wrong