Britney Spears reportedly gets a "thrill" out of stealing the spotlight from Christina Aguilera.
The singers have known each other since their days as child stars in the Mickey Mouse Club.
Decades later, the women are said to be keeping a constant eye on the other's every move.
"[Britney] gets a huge thrill out of eclipsing Christina," a source told America's OK! magazine.
Apparently being the most in shape star is the centre of their current rivalry.
Christina recently shed noticeable weight and has been flaunting a less curvy figure.
She debuted her drastic weight-loss in a flattering pink fit-and-flare dress at the TCA presentation of The Voice late last month.
The 32-year-old Beautiful singer is said to be upset Britney seems to be keeping pace.
"Christina has a chip on her should about Britney," the source continued.
"[Christina] was looking forward to rubbing her weight-loss in everyone's faces - and Britney is at the top of her list. But it's not worked out like she's planned because Britney is back in top shape herself!"
The star was reportedly "really harsh" when Britney was having a hard time in the public eye and put on a few pounds.
Britney, 31, has since slimmed down.
Apparently neither singer was expecting the other to look so good.
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"Neither of them were expecting for the other to debut their hot new bodies in the same week!" the insider revealed.
Britney and Christina have not publicly commented on the report.