Middlesbrough boss Aitor Karanka was confident of defeating Manchester City after surviving their first-half barrage.
Boro goals from Patrick Bamford and Kike dumped City out of the cup.
Karanka, a former Real Madrid assistant boss, said: "At half-time I told my players - and maybe they thought I was crazy - Manchester City are in our hands because the only chances they had in the first half came from our mistakes, and we fixed those mistakes in the second half and we worked better with the ball.
"We didn't lose the ball like we did in the first half and that is why we had this chance."
Karanka's team are enjoying a fine season having risen to second in the Championship.
They also took Liverpool to a penalty shootout in the Capital One Cup earlier in the campaign but Karanka felt this was his most satisfying result.
"Yes of course (it is)," he said. "I have a been a manager just for 20 months.
"We were very close at Liverpool, but the penalties finished our dreams. Today we have beaten one of the best squads in the world.
"It is very important for our club and everybody, but my thoughts are now on Brentford next Saturday because our first objective is the league.
"They can enjoy it today and tomorrow but Tuesday we have to start to think about Saturday because it's an important game."
