Racing stunned Villarreal in the Copa del Rey round of 32 - a significant upset given that Villarreal sit third in the LaLiga standings behind Barca and Real.
"Sometimes you have to enjoy situations, things can change quickly," Flick told reporters ahead of Thursday's round of 16 match.
"The people of Barcelona are fantastic. It's fantastic to live here. We know our responsibility, and we work hard.
"We won the first title, but that's in the past. Racing will want to beat us, and they did very well against Villarreal, who are third. We have to play at our best.
"They will be motivated. They are a young, brave team that plays well... There will be excitement in the stadium when they welcome us. This is the Cup. We have to be ready from the start."
Barca brought back defender Joao Cancelo on loan from Saudi Pro League club Al Hilal on Tuesday for the remainder of the season.
The 31-year-old Portugal international could feature against Racing.
"We have to make a decision. In defence, with (Pau) Cubarsi, Eric (Garcia), Gerard (Martin), Ronald (Araujo)... we're doing things well. Gerard can also play in different positions, and so can Eric," the German coach said.
"He (Cancelo) can give us quality, and he can give us a lot, also in attack, in the final third to get past the opposition. He's a very good option."
Flick also had some friendly words for Xabi Alonso, who left Real Madrid by mutual agreement on Monday, a day after their Spanish Super Cup loss to Barca.
"This is football... I have a very good relationship with Xabi, we knew each other from his time at (Bayer) Leverkusen, and we kept in touch," Flick said.
"I wish him all the best. We have to move on and think about what we can improve and how. But he's a fantastic manager, and I'm sure he has a great future ahead of him."