Chelsea open their campaign tonight against LAFC.
"We had a few days off (after the Conference League final) and some new players have arrived, so when we saw each other three or four days ago at Cobham, the feeling was a little bit like when you return for pre-season," said Maresca.
"But the first thing I said, and the main message since we returned, is that we are not here for pre-season, it’s a big, big tournament."
He continued: "We achieved something important by finishing in the top four in the Premier League, and we won the European competition we were in. But now that is done, finished, and we have to focus on the competition we are in now."
Maresca has warned his Chelsea players they cannot take any of their games lightly.
He said: "We had one competition this season, the Conference League, and the most difficult thing was convincing the players that it was the game of the season for our opponents, so if you don’t prepare in the right way, you can be in trouble.
"Tomorrow we have a difficult game and if we are not ready and prepared, we can be in trouble. So we must prepare 100%.
"And no matter who the other team is, when you are a Chelsea player or head coach, you need to win games. So hopefully, we can start tomorrow’s competition by winning the game."