The Argentinean forward grabbed a brace while further goals from Leroy Sane, Pablo Zabaleta and second-half substitute Kelechi Iheanacho booked a Quarter Final clash away to Middlesbrough.
And after the game Guardiola had nothing but praise for his two-goal hero - as well as offering some words of encouragement for SkyBet Championship promotion-chasers Huddersfield Town.
Guardiola said: "In many ways it is the best performance I have seen from Sergio.
"He was good with the ball - one against one, two against one, he doesn't miss. He scored two goals, created another one and created a lot of good chances.
"When he plays in that level Sergio is unstoppable. We are so happy with the way he played today."
On City's display overall in a contest for which he made only four changes from the 5-3 Champions League win against Monaco, he said: "I'm satisfied with the performance from the players, from the goalkeeper to the striker. A lot of them were really good.
"We are still in the process of growing - I see many good things repeated game by game but we are still far away from being consistent and solid.
"But the last game we scored five goals, and today five again, so I think the fans will have enjoyed how we played and we are so happy to be in the quarter-finals."
On David Wagner's Huddersfield Town, Guardiola added: “I was impressed with them in both halves - they have good quality players”