McCormack told BBC Sport: "Many people didn't give us a chance looking at that Tonga team, who showed glimpses, but we found a way to win. That is up there with the best Scotland have ever had.
"We earned everything we got. Tonga were brilliant for 30 minutes, but we found a way to win.
"In the last 10 we held firm. My heart was in my mouth and I was not sitting comfortably, but we found a way to win. I am very proud to be associated with that."
Scotland face Italy in their next game on Sunday.