Forlan struck in the eighth minute to force home a scrambled effort.
Atletico coach Quique Sanchez Flores, whose side had lost their previous three games coming into Thursday night's match, was also in confident mood for the return meeting.
"It was an intense game, very tiring. A second goal would have completed the match but we can't complain," he said.
"One-nil is a good result, the dressing room is optimistic but we know the game will be decided in the second leg.
"We need to play a good second leg as Liverpool will come out strongly."
When asked if he thought Liverpool's performance was disappointing, Sanchez Flores added: "They probably feel they didn't play a good match but they fought and only lost by a narrow margin.
"Even in their worst matches they score a six out of 10, they never disappoint."