The Borussia Dortmund striker helped his team to a 4-1 win over Jose Mourinho's side with the second leg at the Santiago Bernabeu to play on Tuesday.
It puts the Germans on the verge of a Wembley final, but the much coveted 24-year-old does not believe the job is finished.
He told Polish TV: "Real are definitely strong enough to win the second tie 3-0. But we will not let them do that
"Obviously we regret conceding one goal which came from our mistake. Luckily, we could score four, but we know that the rematch will be a much harder game."
The Pole has received worldwide acclaim for the goals that secured the win, but Lewandowski was keen to play down his performance.
He continued: "This wasn't the game I felt the best in. I have played better games. I had 100% efficiency in front of goal but I feel I have played better games in my career."
But he was playing well enough to volunteer to take the crucial penalty in the second half
"I was scheduled for penalty duty in the Bundesliga, whenever I felt well," he said. "I felt very confident. The most important thing is that I scored it."