Britney Spears and Heather Morris pose on the set of "Glee." Photo: Adam Rose, AP

Britney Spears' appearance on "Glee" last night brought in 13.3 million viewers to Fox -- the highest rating ever for the musical comedy-drama TV series.

Way to go Gleeks!

The singer showed off her newly svelte figure in a fitted white vest and pant set for the "Me Against the Music" duet with Heather Morris. Spears even tweeted, "Awww Heather Morris is sooo cute! Brittany S Pearce. Ha! She was so fun to work with and was really sweet in person."

Later, the paparazzi favorite gave the girls advice in a Cheerios cheerleading uniform. Spears looked every bit the part with messy blond locks, pink lipgloss and pearl earrings. Dressed in two belly-baring looks, the mother of two flashed her toned tummy reminiscent of her glory days as a sassy, Catholic school girl in the video "...Baby One More Time."

Brit Brit also tweeted "Watching Baby brings back so many memories! Feels like yesterday..." Funny girl Lea Michele gained newfound confidence while performing the 1998 smash hit in the notorious uniform and later in an athletic sports bra. Spears makes a super short cameo as Michele's teacher donning clear eyeglasses.

The episode, cleverly titled "Britney/Brittany," featured hits across the 28-year-old Spears' career, including "Toxic," "Me Against the Music," "I'm a Slave 4 U," "Stronger" and breakout single "...Baby One More Time," as our friends over at PopEater report.

Check out the full episode below and leave a comment telling us your favorite Spears style moment from "Glee."