Proving age is nothing but a number, Spears showed off her incredibly fit figure in a lacy white crop top with feathered sleeves, black booty shorts, fishnet tights and thigh-high boots.
Her blond tresses were pulled back into her iconic pigtails from her "…Baby, One More Time" video, taking fans on a nostalgic journey as she performed classic hits like "Stronger," "I'm A Slave 4 ," "(You Drive Me) Crazy," and "Toxic."
ET's Katie Krause was backstage at the festive fête, catching up with Seacrest moments before the big surprise.
"I may have something up my sleeve for her on stage when we get out there," he teased. "It's her birthday, so when you have a birthday, you have to get something with your birthday. It's one of the two things -- a present or a cake."
"It's not a present, it is a cake, but don't tell her!" he added. "I'm going to try and get the whole place to sing, but shh. If she come [backstage], dont tell her."