They lost the playoff against Canada in Montreal on Friday.
"We sent a clear message to our opponents today,” said Canada's non-playing captain, Martin Laurendeau.
"It's certainly a great position to be in up 2-0. It doesn't happen very often and it's up to us to make the most of it tomorrow."
Scholtz, 21, held his own against world number 15 Milos Raonic but eventually lost 5-7, 4-6, 5-7.
Earlier, Izak van der Merwe, the SA team's top-ranked player, went down 3-6, 4-6, 4-6 against Vasek Pospisil in the first match of the tie.
"I didn't play as well as I wanted to, but what's important is that I got the win," Raonic said.
"Vasek won as well and we are in a great position for the rest of the weekend.
"I'll work on my game and be ready for Sunday."
Van der Merwe pairs up with veteran Raven Klaasen in the doubles rubber against Pospisil and Daniel Nestor on Saturday, needing a win to keep themselves in contention for a place in the World Group next season.
The South Africans, already without SA number one Kevin Anderson, were dealt another blow on the eve of the tie when Davis Cup veteran Rik de Voest pulled out with a wrist injury. Sapa