I suffered the same fate today.
After playing one set a day, twice a week to slowly build up my legs, today played 2 really slow, lower level push sets, then 2 fast moving 4.0-4.5 doubles sets. By the time the first point started in the better level set, my third, the legs were totally gone. Just watched as low balls whizzed by, late for every volley and poach, and late for most service returns, as well as out of time for my own serves and weak volleys. Twas nice to play really bad, and not blame my racket in any way, any shots.