Speaking in his pre-Villarreal press conference, as cited by Anfield Watch's Twitter account, Klopp praised the Spanish side and also their manager, Unai Emery.
"We have analysed them properly. I had a lot of respect for Emery and Villarreal before, but I watched them properly and....wow! Impressive," said Klopp.
"Unai is a detail-obsessed manager who prepares for all different situations in the game. They have different ways to press and they react to different results. He is a world-class coach who is doing an incredible job.
"They are 7th in the league, but they have had a really good run of results. They beat Bayern - they beat Valencia - Spain is a very competitive league as well. Unai shows he has a plan for each result, whether 1-0 up or 1-0 down.
"They always have a plan, if they are not winning, they are close to winning and that is really impressive.
"We know that when we go there, it will be an incredibly emotional atmosphere there, so we need to make sure it is like that tomorrow as well. Everyone has to be completely on top. Our supporters are smart.
"There are moments when we will suffer tomorrow night and I know that. We don't need to get nervous but we can make them suffer too. We will work with our result and fight at Villarreal because it means everything to us."
Anfield Watch Verdict
Liverpool definitely do not have an easy task against Villarreal on Wednesday. They are in the Champions League semi-final for a reason, not because of luck.
With Liverpool's high line, Emery will probably tell his centre-backs to play long balls over the top to try and take advantage of the Reds' offside trap.
Whatever happens, Wednesday night is going to be a tough game for Liverpool, and Klopp will have his work cut out trying to break down their defensive system.