How Britney Got Her Body Back

Spears, 27, shows off killer abs in a new tour rehearsal photo posted on her Web site Friday.

In other photos (taken Monday night), the singer (in Lisa Kline J'aime Drawsting Pants) works with contributing choreographers JaQuel Knight and Jose.

Spears has shed 20 pounds in recent months, whittling her waistline to its trimmest in years, thanks to two-hour cardio-and-strength-training workouts three times a week (on top of dance rehearsals).

"It's a nonstop workout," Bally Total Fitness trainer Steven Boggs, who has worked with Spears, said of the routine, which includes moves like shoulder presses and jump squats.

She has also revamped her diet, opting for a 1,100- to 1,200-calorie-a-day meal-delivery plan from Sunfare consisting of three meals (breakfast burritos, barbeque chicken salad and turkey burgers) and two snacks (think cheese and nuts).

"I have had to work hard at getting [my body] back to where it used to be," she said in January's Glamour. "I used to be obsessed with working out ... But I can't get motivated right now because I'm so focused on my music."

She kicks off her world tour, The Circus Starring Britney Spears, March 3 in her home state of Louisiana.