The Blues were a goal up when the Mexican connected with Danny Welbeck's cross with a firm header but the Czech Republic international somehow got a hand to the goalbound effort.
It proved to be a vital save as Chelsea held on to book a semi-final against Manchester City at Wembley later in April.
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"I went as fast as I could and left my left arm to do the job. I tried to cover as much space as I could and luckily I was able to reach it.
"I knew before the cross came that Hernandez was there, so I knew I'd have to be really quick to get there, that's what allowed me to make the save.
"You need to have awareness of the people around you as it helps you make the right decision.
"It was an important moment in the game, I'm sure a lot of people could already see the ball in the goal, so I was glad I got enough on it to push it over.
"I got a few messages from other players and keepers, it's great that people can appreciate the save.
"Even in the first half with the shot from Hernandez, when the ball moved away from me, it was a scary moment and it took a lot of energy for me to kick the ball away, so I’m pleased with the way I played."