The 53-year-old is exclusively performing her classic hits including "Nasty", "Together Again" and "Rhythm Nation" during her "Metamorphosis" gigs at the Park Theater inside the Park MGM hotel and casino for just 18 nights.
Union shared a snap of her posing backstage with the music icon on Friday in Instagram, and wrote, "Opening Night... @janetjackson in Vegas Babyyyyyy so glad we could be there to support! Thank you for inviting us."
Queen Latifah shared a sweet video on Instagram after watching the show, and said Janet "ruled the stage".
"We're so happy to have been a part of it," the actress and singer gushed.
The residency was initially due to run for only 15 dates but three more shows have been added due to popular demand. It is now set to conclude on 17 August.