One of President Obama's top foreign policy advisers was still in critical condition Sunday after 20 hours of emergency surgery that began Friday to repair a torn aorta.

Richard Holbrooke, the president's special envoy to Afghanistan and Pakistan, has received visits at George Washington University Hospital from Secretary of State Hillary Rodham Clinton, whom he was reportedly meeting at the State Department when he fell ill Friday, and Chairman of the Joint Chiefs of Staff Adm. Mike Mullen.

Senior Adviser David Axelrod said Sunday on CNN's "State of the Union" that the ambassador is a "very tough person" who had weathered a "tremendously difficult situation" in Friday's emergency.

"He had an aortic bleed, and many people would have succumbed to that," Axelrod said. "Richard is fighting through it. Anyone who knows him — and I was with him Friday morning before this happened, knows how tough and resilient he is.

"And we're all praying that that quality sees him through now."

Obama called Holbrooke's wife on Saturday to convey that his prayers are with him, according to the White House.

Afghan President Hamid Karzai has also sent his best wishes, wishing Holbrooke a speedy recovery.

Holbrooke, who served as U.S. ambassador to the United Nations during the Clinton administration, is a career diplomat who brokered the 1995 agreement to end the war in Bosnia. But the Af-Pak mission has been fraught with challenges, including sensitive negotiations with Kabul and Islamabad, and it's unknown how Holbrooke's prolonged absence could affect the administration meeting its benchmarks in the Afghanistan war.