Supreme Court Justice Ruth Bader Ginsburg is being treated in hospital after fracturing three ribs in a fall in her office.
The 85-year-old had initially gone home after the incident but was admitted to hospital this morning after experiencing discomfort overnight.
George Washington University hospital said the justice was taken in for “observation and treatment”.
Ms Ginsburg is the court's senior justice, having served for more than 25 years since being appointed by former President Bill Clinton in 1993.
The justice has a reputation as being the most progressive out of the Supreme Court's four liberal judges.
Ms Ginsburg wrote the first opinion of the Supreme Court’s term this year on age discrimination.
This was the third consecutive year in which Ms Ginsburg has authored the first opinion of the session - with Chief Justice John Roberts having previously lauded Ms Ginsburg’s speedy delivery of opinions.
The Supreme Court'ss 5-4 conservative majority was restored last month when the US Senate confirmed Donald Trump's appointee Brett Kavanaugh after a contentious nomination process in which Mr Kavanaugh faced an accused of sexual assault stemming from the 1980s. He denied the allegation.
If Ms Ginsburg were unable to continue serving on the court, Mr Trump would likely move swiftly to replace her with a conservative, further shifting the court to the right. That would have major consequences for issues including abortion, the death penalty, voting rights, gay rights, business litigation and presidential powers.
As the oldest justice, Ms Ginsburg is closely watched for any signs of deteriorating health. She has survived bouts with cancer and undergoes regular medical checkups. This week's incident was not the first time Ginsburg has suffered an injury as a result of a fall - in June 2012, she fell at home and cracked two ribs.
Mr Trump was headed to the Supreme Court on Thursday morning for a formal ceremony welcoming Mr Kavanaugh to his seat.
Reuters contributed to this report
