The 2001-02 CL season was the closest. Liverpool and Man U made it to the quarter-finals, and if they won their brackets, they would have faced in the semifinals, due to the draw being done previous to the start of the second round. And to think, the semis would have featured El Clasico and Liverpool vs. Man U.
https://www.reddit.com/r/soccer/comments/49uwiy/match_thread_liverpool_vs_manchester_united/

