The pop star posted a photo of 8-year-old Maddie on Instagram Monday and wrote, "Need all the wishes and prayers for my niece."
Maddie is the daughter of Spears' younger sister, Jamie Lynn Spears.
The child was steering her ATV to avoid a ditch when she drove into a pond on the family's property in Louisiana on Sunday afternoon, according to a statement from the Tangipahoa Parish Sheriff Daniel Edwards.
Family members rushed into the water to try and rescue the child, who was "trapped and secured by the seat belt and the ATV's safety netting," the statement read. First responders arrived on the scene within two minutes and assisted in freeing the girl, who is reportedly in stable but critical condition, according to the Sheriff.
Jamie Lynn Spears, 25, starred in Nickelodeon's "Zoey 101" as a teen. She stepped away from the spotlight when she became pregnant in 2007.
Now a country music writer and singer, she released an album titled "The Journey" in 2014. That same year, she married voice actor Jamie Watson.
™ & © 2017 Cable News Network, Inc., a Time Warner Company. All rights reserved.