Lance Bass Celebrates Birthday Weekend in Vegas With Nelly

Lance Bass went all out for his 46th birthday … spending the weekend partying in Las Vegas, and getting pulled onstage by Nelly.

The NSYNC singer was at Ayu Dayclub at Resorts World on Sunday when Nelly brought him onstage for a surprise performance … and the crowd went bananas.

TMZ obtained video of the Lance-Nelly gig … and they performed their hit from back in the day, “Girlfriend,” which was a collab between NSYNC and Nelly.