Taylor Swift -- Grinds On Jennifer Lopez During Concert

Taylor Swift + Jennifer Lopez's ass + songs from 2002 = CONCERT GOLD.

It all went down at Taylor's concert in L.A. Saturday night. The pretty-straight-laced singer has been inviting a slew of guests to perform with her on tour: Sara Bareilles, Ellie Goulding, Tegan and Sara ... acts that fit the Taylor mold.

But Saturday she threw that out the window and brought out Jennifer Lopez instead to sing "Jenny From the Block" as a duet ... and even tried to rock some fly girl moves.