Real are on the verge of a fourth consecutive last-four appearance after beating Borussia 3-0 in the quarter-final, first-leg in Madrid last Wednesday, when Lewandowski was suspended.
The 25-year-old returns to lead Dortmund's attack at the Westfalenstadion, with the hosts needing a near miracle to reach the semi-finals.
But no Merengue fan needs reminding of the Poland international's four goals against them in last year's semi-finals, when Dortmund eliminated Real.
"Robert Lewandowski is a fantastic player and extremely dangerous," Ancelotti told Sky Sports. "He is the key player for the type of football Borussia Dortmund play.
"Dortmund will try everything to turn around the result from the first leg. It will be an extremely intensive game and we need a good day."
Ancelotti is waiting on the fitness of Cristiano Ronaldo, who broke off training after only 20 minutes on Monday and missed Saturday's 4-0 win at Real Sociedad with a knee injury.