Doubles match with warmup downline against a notorious player who treats the warmup like a match. I give him a feed he nails a flat ball straight at the baseline, if I get that back he hits another ball as hard as possible, often over the baseline, sometimes at my partners side of the court. His feeds are aimed the baseline flat full pace, often flying miles long or hitting the net.
I give him easy feeds to warmup the volley, I stepup to the net I get the ball nail at me full pace at net height. He isn't the only one who does this, but he is the worse.
How do you deal with assholes who try to win the warmup?
Hmm... i'm not this bad

So i typically like warmup my timing and stroke technique with mini tennis, then move back for easy full stroke shots baseline to baseline, often aiming long, then work steadily increase my swing speed depending on how well my opponent is handling them.
If someone does that to me, firing out the gate, and I can't handle it.. i'll typically make a comment like, "whoa, you have too much juice for me, can you slow it down so i can get more balls back, then we can work up to that speed?".... that usually requires swallowing some pride, but gets the job done... but typically i like banging in the warmup (presuming i'm already warm), as long as they are at least hitting it to me (going long is ok). also, i don't get too bothered about not getting groundies warmup, since I'm going to be at the net 80% of the time anyway.
side story... while practicing server, a new guy i've never met asked me to hit,... good fh, no bh, couldn't move & hit well, and no serve... we rally for about 5m, i fed everything to his fh, but then whenever i give him back an easy sitter at the baseline, he'd put it away (or try)... after 3 times I say, "don't be a d*ck hitting winners off sitters, i'm not your f'ing ball machine. if you want to drill, let's drill, otherwise if you're gonna just hit winners to show me how good you are, get off my court." we ended up having a solid hitting session after that (i avoided his bh) after that.