The singer was on set filming her new video "Work Bitch," but that's not the only reason why she's been toning up.
"She's been a terror lately, working as hard as she can," the Grammy winner's trainer Tony Martinez tells E! News exclusively. "She's using all sports right now to get ready for her 90 minute Las Vegas show."
Although her workouts are less than an hour (about 40 minutes), Martinez says they are grueling.
"It is intense, interval training to keep her heart rate up. In a show, she will be dancing and singing non-stop. In her training, she only has about 10 to 15 second rests between each set," Martinez explains. "She is doing Olympic-style training. This is hard-core stuff. Britney knew she needed to step up her game. She is just drilling it but she likes too! She loves to sweat and she still talks smack to me."
Because the routines (something he calls Trickeration) are so intense, Martinez's main focus is making it fun by using sports, which is why he allows her to play tennis and do soccer drills .
"She loves tennis. She likes to serve. She was serving the ball like Serena Williams!"
In addition to tennis, Martinez likes to fill up her workout time with other sports such as boxing, football and basketball. Apparently our girl Brit's got quite the arm too: "She throws a perfect spiral every time," says Martinez.
She also climbs a 10-foot fence and does several exercises with the trainer's core discs and workout bands (part of his Travel Fit bag, available for purchase next week). Why? Britney really wants to tone up her tummy. "She always wants to work her abs," he shares.
We're exhausted just by reading the routine! Leave it to Britney to do it all with that signature smile on her face.
"She is mentally and physically 100 perfect ready for Vegas. She's so excited to perform and show what great shape she is in," he gushes of his client, adding: "She is working so hard. It is paying off. She is in a really good place and I'm so proud of her."