LAS VEGAS -- Boxer Johnny Tapia remained unconscious Monday on life support at a Las Vegas hospital after collapsing late Friday after he returned home following a standoff with police in Bullhead City, Ariz.

Doctors were running tests to determine why the 35-year-old boxer fell at home, said Trayce Zimmerman, a spokesman for the five-time world champion. His wife Teresa was with him at the time.

Tapia, who was admitted early Saturday to University Medical Center, is being treated for head trauma.

A hospital spokesman said Tapia's condition improved Sunday from critical to serious, and that his family is with him.

Police have said Tapia was involved Friday in a standoff at a house near Bullhead City that resulted in the arrest of two men, including Tapia's cousin. Tapia himself was given a citation and summons charging him with possessing drug paraphernalia.

Tapia, who two years ago was hospitalized for a drug overdose after attempting suicide, last fought Nov. 2 when he lost a 12-round decision to featherweight Marco Antonio Barrera.