Enter Sociedad Brilliant in the match after nine consecutive victories in all competitions and came close to scoring in the 29th minute when Japanese teenager Takei Kubo hit the crossbar.
But his hopes of achieving a surprise in Camp Nou dissipated while Striker Bryce Mendez was sent off in the 40th minute for a heavy tackle on Sergio Busquets.
It was Barcelona The better side in most of the match, but scored only from a wonderful counterattack, which was crowned by Dembele, man of the match in the 52nd minute.
The speedy winger broke out from the right after receiving an excellent long pass from his French compatriot Jules Conde and fired an unstoppable right-footed shot high into the net.
The visitors could have equalized after seven minutes but Alexander Sorloth bizarrely missed a chance just steps from goal when he shot into the sky after Kubo’s pass.
And after Gabi hit the crossbar after a wonderful cross pass from Dimbley In the 68th minute, Real pressed late and could have scored from at least three chances, but goalkeeper Marc-Andre Ter Stegen preserved the victory.
Barcelona defender Ronald Araujo told Movistar Plus, “We had difficulty after the numerical shortage, because they closed the spaces tightly and defended fiercely.”
He added, “I am sure that coach Xavi will admonish us. We could have decided the match early. We must correct the mistakes now and move forward. I am happy that we won at home and continued a good run.”
Also in the quarter-finals, Sevilla plays against Osasuna, and Real Madrid meets its rivals Atletico Madrid Valencia and Athletic Bilbao will also face each other on Thursday.