Karnezis will be in goal for Fernando Santos' men in their tournament opener in Belo Horizonte, and noted Colombian trio James Rodriguez, Carlos Bacca and Teofio Gutierrez as the men for Greece's defence to watch.
"It's a dangerous midfield line-up with James, Bacca, 'Teo'. We have to stay focused," Karnezis said.
"We have to be in good condition on game day and totally focused. If we do this, we will not have any problems.
"No one can have arrogance; cannot talk and talk, what we have to do is show all on the field of play."
The 28-year-old Granada custodian said the absence of Colombia striker Radamel Falcao was 'sad', but it would not be to the detriment of Jose Pekerman's side.
"It's sad because as we would all like to play the World Cup as a player but I understand their position," he said.
"But we must see that Colombia has top players and do not rely on one player. We are ready for everything."
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