Medical personnel rushed to the 2002 World Cup player's aid early on Tuesday after he collapsed with suspected heatstroke. His JFL team Matsumoto Yamaga later said he had gone into cardio-respiratory arrest.
Doctors put 34-year-old Matsuda on an artificial respirator and described his conditions as "extremely critical."
Yamaga manager Yoshiyuki Kato told a news conference on Wednesday: "He was stable this morning but there was no change in his condition."
Former Japan team-mate Shunsuke Nakamura rushed to the hospital to see Matsuda, spending 20 minutes looking through the window of the intensive care unit.
"I heard he'd collapsed but didn't hear any details," the Yokohama midfielder told Wednesday's Nikkan Sports newspaper. "But I saw him and believe he will recover."
Former Japan captain Yuji Nakazawa, part of Yokohama's back-to-back title winning teams of 2003 and 2004 alongside Matsuda in defence, was stunned by the news.
"I can't believe it," he told the Sankei Sports. "I had only heard he fainted. "I hope he recovers consciousness quickly and we can hear his voice again soon."
Doctors suspect the player may have suffered a heart attack after he arrived at hospital unconscious and not having responded to heart massage, his club said.
Record temperatures across Japan have caused a rise in heatstroke cases with 43 deaths in the two months through the end of July, Kyodo news agency reported.
"Matsuda is the sort of player who has enormous influence over a team," said Yamaga president Hiroshi Otsuki. "I and fans all over Japan am praying for his full recovery."
Matsuda has played 40 times for Japan and made 385 appearances for Yokohama F-Marinos.
