Atletico are currently top of the La Liga table, three points clear of local rivals Real Madrid.
Despite Chelsea's status as favourites to reach the final, Cech believes that Diego Simeone's team will provide a stern test over two legs at the end of the month.
"They're a team who have been together for some time, and they have had success since Diego Simeone took over," he told the club's official website.
"They're a very hardworking team, very organised, disciplined and emotional. It's always difficult to play against them but in the Champions League semi-finals there are no choices among the four teams - you can't really pick who you want to play."
Chelsea travel to Spain for the first leg on April 22 before welcoming Atletico to Stamford Bridge on April 30.