City won the quarterfinal 2-0 via a Bernardo Silva brace.
Doku said: "It felt good, I'm happy with the performance of my team, the performance of myself.
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"I feel like I'm getting better and better. We scored important goals, we defended well, attacked well, very happy.
"We were in control but you never know with the counter attacks, they have some fast players and they want to run in behind. The (defenders) did well today."