I hit with Jesse Witten as warmup for a ProAm mixed doubles (Mike Yani was my partner) and it took Jesse a year to recover from the damage I did to his game.
I hit with Horia Tecau for about 15 minutes in 2006 before Ilija B. kicked us off the court. I was sucking wind like I had just run a mile. He hit the heaviest ball I had ever dealt with and it nearly killed me (in between laughing our heads off at how badly I suck at tennis).
Justin and Jan-Michael have both promised to hit with me 'some day' so I've got that to look forward to, but both have said they will only do it if I start playing regularly again which I haven't done in over a year. Perhaps I'll dust off my racquet when the rain stops.