Barbra Streisandand Ariana Grande blended classic and contemporary voices for one helluva surprise performance, and only 2 words come to mind --like buttah!!!The legendary singer was in the middle of performing Tuesday night at United Center in Chicago when, out of nowhere, Ariana -- who idolizes Babs, btw -- strolled onstage. Naturally, her surprise appearance drew a roaring ovation as they launched into Barbra’s 1979 hit duet "No More Tears (Enough is Enough)." AG sang the legendary Donna Summer’s part.



You can tell Ariana’s THIS close to losing it herself from excitement. Her review of the performance was pretty simple -- she wrote on social media, "Best night of my life."After exchanging pleasantries, Ariana cracked a joke, "I’m gonna go pass out now. You’re just gonna find 10 pounds of hair."

The "7 Rings" singer later posted a picture with Barbra and captioned it, "Cherishing this moment forever." It seems Ariana decided to stick around in Chi-town after her Sunday performance at Lollapalooza.

She had to wait a couple days to play out her dream ... but clearly, it was worth it.