The singer was seen practicing her number for the highly-anticipated performance ... as she was photographed singing on golden steps alongside the Seine in Paris. Gaga pulled out all the stops for her gig, too ... as she performed in a chic all-black ensemble next to dancers and a full jazz band decked out in similar monochromatic looks.

Gaga and her dancers rounded out the choreography by integrating over-the-top pink feathered fans ... reminiscent of the famed Moulin Rouge cabaret.

However, there was one thing noticeably missing from Gaga's rehearsal ... Celine Dion.

As TMZ previously reported ... Gaga and Celine are set to perform "La Vie en Rose," a French classic made famous by Édith Piaf.

Celine will reportedly be wearing all Dior for the performance, where she'll be decked out in a pink and black feather cape ... which goes along with the theme at Gaga's rehearsal.

Celine's absence from the rehearsal may mean nothing ... because we don't know exactly how Opening Ceremony producers plan to integrate their performances.

As NBC's Olympics host Mike Tirico told Hoda Kotb and Savannah Guthrie Friday on "Today" ... the opening ceremony is taking place across different spots in the city. So, it's possible Celine will sing from a different locale than Gaga during their duet.

Celine has also been seen out and about in Paris ahead of the Olympics' opening ceremony ... where she was filmed waving to crowds from the roof of her town car -- so, y'know she's about to bring it for her fans.