“Free Britney!” Nick Carter declared before launching into a seductive cover of Britney Spears’ “Toxic” in Las Vegas on Thursday night.

The musician and his Backstreet Boys bandmate AJ McLean, along with *NSYNC’s Joey Fatone and Boyz II Men’s Wanya Morris, kicked off their four-night engagement “The After Party” to a packed, masked crowd at The Venetian Resort’s intimate Sands Showroom.

The ode to former Las Vegas residency queen Spears kicked off a run of crowd-pumping covers, which saw Fatone shake it off to Ricky Martin’s “Livin’ La Vida Loca” before launching into late EDM legend Avicii’s “Wake Me Up”, accompanied by Morris’ soulful harmonies.

While few may have expected the likes of Avicii on the set list, the track reflected the musical curveballs Fatone teased while announcing the mini-residency in June. And the surprises kicked in as soon as the lights went down, with Fatone and Carter (who previously performed “Toxic” on “The Masked Singer”) making their entrance in unexpected ways before the four musicians launched into each of their bands’ hits – *NSYNC’s “Pop”, Boyz II Men’s “Motownphilly” and Backstreet Boys’ “Larger Than Life”, the latter getting the few who still remained in their seats up on their feet.

By the time the group, accompanied by a live band, launched into Bell Biv DeVoe’s “Poison” amid a perfectly synchronized sea of waving arms, there were few signs of a pandemic beyond the mandatory masks being worn.

Reflecting on pre-Covid days, the singers swapped stories about their shared experiences on “Dancing with the Stars” Carter, 41, sharing that he competed to lose weight, Morris remaining perplexed that he lost to Nyle DiMarco and McLean, 43, declaring he and his dance partner, Cheryl Burke, “got f**ked.”

“Honest to God, I love Nelly to death, but should it have been me in the final four or Nelly?” McLean quizzed the audience, who roared back their support.

Taking a trip further back down memory lane, Morris, 48, sat down on the couch with Fatone, 44, to discuss his musical parents and spending his childhood inspired by television theme songs.

“I lived in the projects, so we would sit on the floor [and] I would clear the roaches and TV shows would come on,” Morris recalled. “Because there was no TiVo or rewind [button], you literally had to learn a song from the TV show every day or every week [when] the song would come on. The first one was ‘Welcome Back, Kotter.’” Morris then launched into the opener of the seventies sitcom’s theme song.

The group also performed the “Cheers” theme song along with other nostalgic television hits and while energy began to wane in the theatre, Fatone fired things back up by bringing out an old high school acapella group peer – Fonsi, who performed his 2017 smash “Despacito” with Carter, McLean, Fatone and Morris.

Invited to stick around, Fonsi then belted out Journey’s “Open Arms” with Fatone, grabbed a drink at the on-stage bar then joined in on the Backstreet Boys’ “All I Have to Give”.

However, Fatone interrupted before Carter could get the first verse out.

“You sing this song and I hear it every time – why do you sound like you’re constipated?” he quizzed Carter about his vocals in the 1998 track.

“Joey, first of all, I was 15-years-old when I recorded that song,” Carter responded. “Second, I hadn’t gone through puberty yet. And third, f**k you Joey!”

“I hate this song because I wish we had sung it,” Fatone offered.

Getting the last laugh – as well of plenty of booing – Carter quipped, “Well you know what? At least, Joey, I had some lead parts.”

Carter later conceded that Fatone deserved more credit in *NSYNC and should have had “a helluva lot more leads” in the group’s hits.

Although the vocalists fed into the supposed Backstreet Boys and *NSYNC rivalry which continues to make headlines today, it was the Boyz II Men ballads which once inspired both boy bands which proved a highlight of the night, the foursome powerfully belting out “On Bended Knee” and “End of the Road”.

“It’s because of that right there,” McLean told Morris about his musical inspirations after the clan performed another Boyz II Men favourite, “I’ll Make Love to You”. “I would sit in my bedroom listening to you. Just you!”

The audience agreed. “Wanya gave me goosebumps!” one male enthused while leaving the show. “This was way better than I thought it would be.”

“That was my first concert since COVID and damn it’s a good first,” exclaimed another.

Fonsi returned for another performance of “Despacito” on Friday night, before Morris’ bandmate Shawn Stockman joined the group to perform a medley featuring Bruno Mars’ “Locked Out of Heaven” and Lenny Kravitz’ Are You Gonna Go My Way”.

The excitement was all too much for one fan who crashed the stage before promptly being removed by security.

The run of shows comes following June’s “Back-Sync” debut (which saw McLean, Carter, Fatone and his bandmate Lance Bass perform at “Bingo Under the Stars” at The Grove in Los Angeles) as well as McLean’s joint performance with *NSYNC’s Chris Kirkpatrick and 98 Degrees’ Jeff Timmons at a Las Vegas fundraising event for the Victoria’s Voice Foundation.

When announcing “The After Party” in July, Fatone told Variety the idea was partly inspired by his “Joey Fatone and Friends” concerts at the 2019 Epcot International Food and Wine Festival. He said the concept was further influenced by the Rat Pack.

“We wanted to do something with the idea of the Rat Pack meets the Pop age now — different artists coming together and doing a show that’s fun, but also takes you through a bit of history,” he explained. “But it organically also turned into more of a celebration of our bands and a homage to pop culture.”

The concerts come just days before The Venetian’s Sands Showroom hosts another musical mash-up – with New Kids on the Block’s Joey McIntyre teaming up with Debbie Gibson for a residency which kicks off on Thursday 28 August.

Meanwhile, McLean and Carter will be back in Las Vegas in November and December for the Backstreet Boys’ holiday residency, “A Very Backstreet Christmas Party”.

Launching shortly after the boy band debuts their first Christmas album, the 12-show run will see the quintet return to Planet Hollywood Resort & Casino’s Zappos Theater, where they previously enjoyed their record-breaking “Larger Than Life” run.

The group is expected to perform hits, holiday classics and new Christmas material.

“The After Party” meanwhile continues at The Venetian on Friday, Saturday and Sunday with tickets available at Ticketmaster.