Olimpic arrived to the match with the nine-time European champion on an unbeaten run in the third division. And after Madrid's lackadaisical start, it appeared to grow in confidence it could hold its own and ended up having relatively little trouble recording its eighth straight game without conceding a goal.
Olimpic's coach, Antonio Aparicio, was just proud of his team.
"It was a party for everybody. My team gave a good showing and worked so hard. What more can we ask for?" Aparicio asked. "We can always say that we drew with Real Madrid."
Ancelotti said Madrid's players were not accustomed to playing on the artificial turf.
"It wasn't a great game, but it wasn't too bad," Ancelotti said. "I am satisfied. We don't have to be worried."
Also, Celta Vigo beat visiting Athletic Bilbao 1-0 to take the advantage in their tie.
Levante became the first topflight team to lose to a lower-division opponent after falling 1-0 at Huelva Recreativo, while Getafe drew 1-1 at second-division Girona.
With various veteran stars injured or resting, it was a night for some of the more promising talents to shine in the wins of Atletico and Celta.
Atletico coach Diego Simeone left strike duo Diego Costa and David Villa on the bench for the first hour, opting for a starting 11 with just four regular starters and featuring youngsters Oliver Torres and Javier Manquillo.
It was still more than enough to overpower Sant Andreu.
"This team doesn't let up for any game," said Manquillo, Atletico's promising 19-year-old right back. "We considered this cup match was like any other and we always respect our rival, no matter who it is."
Raul Garcia almost scored just seconds into the match at Narcis Sala Stadium in Barcelona's northern suburbs. But he succeeded in the 13th after he volleyed Filipe Luis' out-swinging cross unmarked at the far post.
Arda Turan then slotted in Manquillo's back pass in the 20th, and he scored again from a series of tightly knit passes in the 56th that included two audacious back-heeled flicks by Adrian Lopez and Garcia to set up the Turkey international. Villa sealed the victory in the 84th from a pass by Diego Costa.
Celta Vigo also turned to a youth player to edge Bilbao in the day's only matchup between first-division teams.
After both sides went close several times at Balaidos Stadium, Celta striker Santiago Mina celebrated his 18th birthday by scoring the follow-up of his own initial strike that Bilbao goalkeeper Iago Herrerin did well to block but couldn't hold on to in the 71st.
Celta 35-year-old forward Mario Bermejo went on as a late substitute in his first action since tearing his left knee ligament in May.
Recreativo defender Ruyman Hernandez headed in the winner one minute into stoppage time to down Levante.
Getafe forward Ciprian Marica used a bicycle kick to fire in a poor clearance in the 38th, canceling out David Timor's free kick that gave Girona the lead in the 16th.
