Berdych had four breaks in his last six receiving games in taking a 6-1, 4-6, 6-1, 6-4, 6-4 decision over Juan Monaco.
The teams will be looking for the their doubles teams Saturday to give them the edge going into Sunday's reverse singles, when Berdych and del Potro open play and Monaco goes against Stepanek in the final pairing.
The doubles match has Czechs Ivo Minar and Lukas Rosol going against Argentina's Carlos Berlocq and Eduardo Schwank.
The winner of the series goes to the Davis Cup finals Nov. 16-18 against either the Spain or the United States.
Wins Friday by David Ferrer and Nicolas Almagro put Spain, which beat Argentina in the 2011 Davis Cup final, up 2-0 over the Americans.
Del Potro dropped serve just once in beating Stepanek Friday. He won two-thirds of the points on serve and 46 percent when Stepanek served.
Berdych turned his match with a break in the 10th game of the fourth set that evened the match. He had two more breaks in taking a 4-0 lead in the fifth set. Monaco had a late break of his own to get to 4-5 but Berdych finished things off with a break at love.