The Sharks utility back, who had been a controversial selection ahead of the Bulls' inform pivot Morne Steyn, was one seven front-line players substituted during the closing stages and was forced to watch from the sidelines as the Lions closed the gap to 26-21 with minutes remaining.
"It was very, very scary," said Pienaar, after he watched tries from Tom Croft and Mike Phillips drag the tourists back to within touching distance at the ABSA Stadium.
"We let them back at the end. The Lions played really well at the end and broke the line a few times.
"The guys showed a lot of character to keep them out just before they got over the try-line.
"It was very physical. The Lions have got great backs. Obviously we needed to work very hard in defence to keep them out."
The Lions face the Boks in the second Test in Pretoria next Saturday, with Pienaar expected to play another large part in proceedings as South Africa attempt to put the series to bed with a game to spare.