City were 2-0 up and cruising thanks to goals from Phil Foden (23) and Erling Haaland (23) in a dominant first half, but Rodri's (27) red card in the first minute of the second half saw the roles reversed and the home side had to battle hard for the win.
"An exceptional game in the first half, exceptional game in the second for our resilience and our work," Guardiola said.
"We played, I would say, 51, 52 minutes 10 against 11, so it was not easy, but we were fantastic. I thought the way we played in the first half was amazing against a difficult side for the physicality, the pace they have up front," he added.
The win was City's sixth out of six games in the league so far and though few would have expected Forest to take anything from the game beforehand, manager Steve Cooper (43) was left disappointed that they had not done so once Rodri was sent off.
"It can go both ways then when you're 2-0 down. To be fair to the guys, they really stuck to the task and things changed obviously with the red card," Cooper said.
"Then we became the team that was pushing and pressing... I would have liked us to have shown a bit more quality in the final third."
City stay top of the standings as the only side not to drop points so far this season, while Forest are 10th with seven points from their six games.
Check out the full report from the Etihad Stadium with Flashscore.