Liza Minnelli surprises fans with touching tribute to mother Judy Garland

This article originally appeared on PEOPLE.com.

Music fans at the San Francisco venue Feinstein's at the Nikko were in for a treat on Friday, when Liza Minnelli made a rare, unannounced appearance — and performed a touching tribute to her mother, Judy Garland.

The 70-year-old Oscar winner duetted with Michael Feinstein on "I Love a Piano," the Irving Berlin tune from Garland and Fred Astaire's 1948 film Easter Parade.

Minnelli was introduced to the sold-out crowd by her sister, Lorna Luft, 63, who recently commemorated the 47th anniversary of her mother's death by performing Garland's signature song, "Over the Rainbow," for the very first time ever.

In her introduction, Luft touchingly told the crowd that Minnelli had always been there for her throughout her life — including a recent bout with cancer, that Loft said she’s since conquered.

Minnelli sounded as strong as ever, with her powerful voice on full display in the songs high notes.

And of course, the energetic performer didn't shy away from the laughs. Midway through her song, she stopped the number when her shoe came off — and laughed with an audience member who came to her rescue.

Her performance came at the encore of a night dedicated to Garland's music, in which Feinstein gave his take on Garland classics like "That's Entertainment," "Just in Time," and "You'll Never Walk Alone."

Luft joined him for her take on "San Francisco" and "The Man That Got Away" — and duetted with Feinstein for a "Happy Days Are Here Again" / "Get Happy" medley, made famous by Garland and Barbra Streisand in a 1963 television performance.

Minnelli watched the whole show from the audience, dressed in an all-black with a matching newsboy cap.

She had such a good time at the Garland salute on Friday that she returned to the show — and to the stage — on Saturday night as well.

Fans were just as surprised.