The Blues were 1-0 down at half-time but came back to put five goals past their opponents in a pulsating second half at Elland Road.
Juan Mata, Branislav Ivanovic, Victor Moses, Eden Hazard and Fernando Torres were all on target in a dominant second-half display from the visitors, who now face Swansea in the last four.
Benitez, whose side flew back from Japan at the start of the week following their unsuccessful Club World Cup campaign, told Sky Sports: "We were looking for character and at half-time we talked about this.
"After coming back from Japan, the jet lag... changing some players because the rest were a little bit tired... the reaction in the way that we played was amazing."
Despite losing the Club World Cup final to Corinthians, Benitez insisted the trip to the Far East had been positive in terms of bonding.
He said: "It's always important to spend some time together and we had plenty of time there, talking about everything. It was really good.
"Here we have to play eight games in 24 days so you don't have too much time to prepare things and to train and coach the players properly, so I'm really pleased with the time we had together, because you know the players a little bit better."
Looking ahead to the two-legged semi-final, Benitez added: "Every competition is important for us. A top side has to try and win every competition and now we are in the semi-final, so very close to Wembley.
"We know Swansea are a good team. When you get to the semi-final of any competition you have to do well.
"I think they deserve to be here but hopefully we can do what we did today and we will be in the final.
"They like to play football, they like to play on the floor. We have to do well and also try to keep the ball, so I think it will be interesting."
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