The 'Toxic' singer announced in June that she was to add 20 more dates to the North American portion of the jaunt.
She performed a 90-minute show at Mandalay Bay, which was attended by Barry Manilow and Lance Bass, People reports.
The former 'N Sync member was given a lap dance by Spears during her rendition of 'Breathe On Me', while he sang along with her during the routine.
The popstar was spotted shopping in the area before the gig at Mirage's Sugar Factory with boyfriend Jason Trawick and her two sons, Sean Preston, 3, and 2-year-old Jayden James.
"She spent about $3,000 (£1,880) and walked out of the store with ten bags," a source said. "She got a bunch of candy and shirts for her kids and got a bunch of the couture lollipops to give to her backup dancers."
Spears recently confirmed that she will play several shows in Australia in November.