I have heard rumors saying that Rosol will play on Friday against Del Potro to save Stepanek´s energy for Saturday or Sunday.
I checked DC rules
(pages 22-23), and it states you can substitute player in first day's matches only because of illness/injury or for disciplinary reasons, and that player can't play anymore in that tie. So obviously Stepanek must now play against DelPo. So it would've been better to nominate Rosol as second singles player, and if he plays badly, then substitute him with Stepanek for Sunday, as it's possible to substitute a singles player for Sunday for 10 minutes after 2nd rubber.