Samir Nasri then picked up the ball 25 yards from goal and hammered a shot against the crossbar with the goalkeeper beaten.
Despite looking confident on the ball, Plzen were vulnerable at the back and Nasri again missed a chance for the opening goal. Sergio Aguero ran down the right-hand side before pulling the ball back into the box. Nasri had just the keeper to beat, but put his shot wide.
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Aguero scuffed an effort wide of the post after he had shown good skill to work a shooting position on the edge of the box.
Despite City's pressure, Plzen almost took an unexpected lead. Frantisek Rajtoral worked his way into the box, but with defenders closing him down, his strike across goal was missed by everyone.
However, Rajtoral was soon the villain as he handled Aguero's cross and the referee pointed to the spot. The Argentine striker took the kick himself, sending Kozacik the wrong way for 1-0.
Plzen grabbed an equaliser with just minutes of the first half remaining, however. Tomas Horava received the ball 25 yards from goal, steadied himself, and then fired a shot into Joe Hart's top corner.