The Argentine heaped praise on the Basques for their 'positive' approach at the Vicente Calderon and is anticipating a tough test in the second leg at San Sames
Diego Godin scored the only goal of the Copa del Rey quarter-final, first leg at the Vicente Calderon, the defender heading home just four minutes before the break.
Simeone was enthused by his side's battling performance but praised Athletic for taking the game to the Rojiblancos.
"This was a classic cup game, that is competitive, in which the two teams play for a positive result," Simeone told reporters.
"A 2-0 loss would not have suited Bilbao but a 1-1 draw would so they went for it.
"We didn't expect to see a very open game, perhaps we thought it would be more open in the second one with the result to play for, but not in the first cup games.
"From the experiences I've had, the first legs are always much more studied, but I don't think that was the way tonight."
Indeed, Simeone knows that Atletico still have a lot of work to do at San Mames, particularly as the Liga co-leaders face a trip to Rayo Vallecano this weekend.
"After Sunday night, we will prepare for the second leg in the best way because it will be a tough match in a beautiful environment to play football with a team that is strong," the former midfielder said.
"Bilbao is having a very good season so far, they are fourth in the league and playing good football.
"To keep that place in the league would be exhausting, so we must be attentive and make sure we are focused.
"We have to be aware of what they need to do."