Former Japan defender Naoki Matsuda, who collapsed during a morning practice and was confirmed to have suffered a cardiopulmonary arrest, was in critical condition Tuesday night.
Matsumoto Yamaga club president Hiroshi Otsuki told a news conference at Shinshu University Hospital in Matsumoto that Matsuda, 34, has been put on an artificial respirator at the hospital to maintain circulation of his blood, but his condition "remains grim."
Doctors think he may have suffered a severe heart attack.
Matsuda, who is suspected of having suffered heat stroke after the practice session began at 9:30 a.m. on Tuesday, was unconscious on his arrival at the hospital and has shown no response to heart massage, the Japan Football League club said.
Matsuda played for the J. League's Yokohama F. Marinos from 1995 to 2010, taking part in 385 games, before joining Matsumoto Yamaga.
He played 40 times for the national team, including the 2002 World Cup cohosted by Japan and South Korea.