It’s hard to believe that just two summers ago Backstreet Boys were on top of the boy-band world. In teen-pop years, it seems at least a millennium since they ceded the crown to the younger, wackier ’N Sync. And last Friday, at the first of five scheduled FleetCenter shows, it wasn’t even clear whether the once-formidable Boys had opener Shaggy beat in Top 40’s ongoing popularity contest. Then on Monday, the group cancelled their remaining three Boston appearances and postponed their tour for a month so that A.J. McLean can get treatment for clinical depression and alcohol abuse. But if Backstreet are on the way out, they’re not going quietly: Friday’s show ran two hours strong, with all the heavy artillery, implied raunch, and cuddly moments a little girl (and maybe even her chaperones) could hope for.

’N Sync may be opting for edgier musical collaborations these days, but at the FleetCenter Backstreet easily held their own in terms of charisma and high-powered production values. A fiery space battle heralding their entrance was followed shortly by a crowd-pleasing one-by-one introduction segment. Big blond dude Nick Carter got the Justin Timberlake star-among-stars treatment, and he played into it nicely by licking the video monitors and reaching into the crowd every chance he got. Kevin Richardson may have looked a little ridiculous with his new cornrows, but in general the Boys were as sincere and good-humored as ever.

Backstreet’s current tour has one major advantage over ’N Sync’s: no unfamiliar new material to kill the momentum. The quintessential teen-pop anthem “I Want It That Way” was an early highlight; it was followed by a mid-show ballad bonanza that took place on an auxiliary stage at the back of the arena. Carter stripped down to a wifebeater and took a turn behind the drums on “The Answer to Our Life,” the group’s breezy songwriting debut. McLean became the first and only Boy of the night to go shirtless during “The Call,” which sent fans home with one final burst of flames.

Still riding high on the success of last year’s Hotshot (MCA), nappy-haired dancehall loverman Shaggy had fans smiling and waving their arms throughout his 45-minute opening set. “Where my angels at?” he yelled as an introduction to “Angel,” then brought a young girl out of the audience and serenaded her on stage. Things got a little more mischievous during the players’ anthem “Wasn’t Me” (specifically, the flamboyant gyrations and extra snare hits that accompanied the lyric “bangin’ ”), but for the most part Shaggy delivered the kind of wholesome family fun Backstreet fans have come to expect.