The Boks started the match with serious intent, scoring five tries in the first 24 minutes, which paved the way for their 33-0 halftime lead and built on this with an impressive second-half showing to finish the match with 11 tries in addition to a penalty try.
"One can't plan for a good or bad start, but you can plan to start with intensity, and I thought we had massive intensity today, which I'm happy about," said Nienaber.
"We showed a lot of respect for Romania this week, and as a team there were a few things we wanted to get out of the game.
"One of those was to give some guys an opportunity in the other positions they cover, such as Faf (de Klerk) at flyhalf, Deon (Fourie) and Marco (van Staden) at hooker and Grant (Williams) at wing, and we were able to do that."
Nienaber added: "All credit to Romania, there will be some sore bodies in our team tonight."
Nienaber said one of the areas they would look to improve on before facing Ireland in Paris on Saturday, 23 September, was finishing their opportunities on attack.
"Cobus (Reinach) and Makazole (Mapimpi) scored three tries and Grant (Williams) two, among the tries we scored," said Nienaber.
"But there were also a few we missed, so we'll have a look at that and try to fix it when we return to the field on Tuesday."
As the Boks' defence guru, Nienaber praised the team for their tenacious defence to deny Romania a single point in the match.
"We gave Romania a lot of attention from a defensive point of view this week, and we worked really hard to ensure we were well prepared for them," he said. "The players put good intensity and effort into that area of the game, and I'm pleased with that."
Vincent Koch suffered an injury in the warm-up, which saw him being withdrawn from the match-day squad and Frans Malherbe being drafted onto the replacements' bench, with Trevor Nyakane starting at tighthead.
Nienaber said: "He hobbled off in the warm-up and we think it's his knee. But I have no idea how bad it is. The doctor will assess the injuries tonight and we'll have a better idea later."
The Springboks will travel to Paris on Monday for their third round Pool B match against Ireland on Saturday.