From that point Palace regained a degree of control, with manager Oliver Glasner able to give Kaden Rodney some pitch time, in place of the returning Richards after he completed 45 minutes.
With just five minutes remaining of the first game, Mainz finally beat Henderson with Weiper reacting quickest, after the Palace keeper kept out a powerful initial effort.
That was how it finished, with an entirely changed Palace XI set to take to the field for the second fixture at 14:20 BST.
Palace XI: Henderson, Richards (Rodney, 45), Lerma, Jemide, Muñoz, Wharton, Hughes, Mitchell, Sarr, Eze, Mateta.
Subs not used: Matthews (GK), Cardines, Edouard.
Mainz XI: Zentner (GK), Mwene, Sano, Nebel, Amiri (C), Bell, Hollerbach, Caci, Da Costa, Kohr, Weiper.
As It Happened