After losing the first set, the Norwegian player staged a comeback to claim a narrow 6-7, 6-4, 10-8 win over Cerundolo, also coming from behind in the deciding 10-point tiebreak.
At his post-match press conference, he was delighted to have given Team Europe a winning start, but saw plenty of room for improvement.
"Yes, it was," he said in response to a suggestion that it was a nervy performance.
"I wasn't finding, particularly, my second-serve returns that well. I had a plan to try and go around and hit as many forehands as possible, and I wasn't able to get around the ball, and mishit a few and hit a few long.
"A little bit, maybe uncharacteristic, as you said. When I play my best, I don't make those mistakes. Somewhat nervy, a little bit rusty.
"I haven't really played a match at this level in a few weeks. Montreal was the last match I played at a full high level, and you feel it. Every player feels this way. The feeling of playing good tennis is fresh; it can come and go fast.
"Hopefully a win like today can show me that even without my top tennis, I can fight the matches and win against good players."
Carlos Alcaraz was seen giving Ruud tactical advice during the match, and the Norwegian player said that having coaching and encouragement from his teammates and vice-captain Tim Henman "helped a lot."
Speaking in the same press conference, Henman discussed his approach to analysing and coaching Team Europe's players, saying that he watches the matches in a similar manner to when he's commentating, and focuses on the opponent.
"I mean, I've got a lot more vested interest, obviously, watching these matches, because I'm part of the team, but I'm always looking strategically when I'm commentating," he said. "Who's doing what and why is one person winning? What are the strengths and weaknesses?
"Having been working in the media for the last sort of five years, I'm seeing a lot of these players play, so there aren't too many secrets or things I haven't been able to watch.
"The coaching element is not teaching. Coaching is about strategy, about game plans and trying to help with the individual matchups, because you only have to be better than one player, and it was very close today, but Ruud was able to be better than Cerundolo, and that's why we won."