Way back when there was no rule at all about rating discrepancy, I played a 9.0 match in which our opponents were a 5.5 man and a 3.5 woman. IIRC, he was also the top-ranked player in his nat'l age group at the time. It was pretty hilarious.
We managed to win a set by breaking his serve the first time ... just hit every ball back to his partner, who flubbed it. But I think that was the last time he let us touch his serve all day.
We played pretty well, all things considered, but the most difficult thing is that you have to think way too much, because trying to hit every single ball (and I do mean EVERY SINGLE ball) to one person makes you hit some really unnatural shots, shots you would never ever hit in a regular match. There is no flow, no letting go and just playing. By the third set, we were kind of a mess.
But I'd never played a real match against an opponent as good as he was, so it was fun in that way.