He blamed his disastrous debate performance on a "bad episode," saying there were "no indications of any serious condition".
"Look, I have a cognitive test every single day," Biden told ABC's George Stephanopoulos, referring to the tasks he faces daily in a rigourous job.
"Every day, I have that test. Everything I do. You know, not only am I campaigning, but I'm running the world."
The 81-year-old Biden made it through the 22-minute interview Friday without any major blunders that would inflict further damage to his struggling candidacy.
But it appeared unlikely to fully tamp down concerns about his age and fitness for another four years, as well as his ability to defeat Donald Trump in November.
I'm not letting one 90-minute debate wipe out three and a half years of work.— Joe Biden (@JoeBiden) I'm staying in the race, and I will beat Donald Trump. pic.twitter.com/5VZHf4N4xjJuly 5, 2024
On Saturday, another Democratic lawmaker joined close to a half dozen in saying Biden should not run again.
Representative Angie Craig of Minnesota said that given what she saw and heard in the debate, and Biden's "lack of a forceful response" afterward, he should step aside "and allow for a new generation of leaders to step forward".
Craig posted one of the Democrats' key suburban wins in the 2018 mid-terms and could be a barometre for districts that were vital for Biden in 2020.
The interview left Biden in a stand-off against a not-insignificant faction of his party with four months to go until election day, and with just weeks until the Democratic National Convention.
The drawn-out spectacle could benefit Biden's efforts to remain in the race by limiting the party's options to replace him.
But it also could be a distraction from vital efforts to frame the 2024 race as a referendum on Trump.
During the interview, Biden insisted he was not more frail than earlier in his presidency.
He said he undergoes "ongoing assessment" by his personal doctors and they "don't hesitate to tell me" if something is wrong.
"Can I run the 100 in 10 flat? No. But I'm still in good shape," Biden said.
As for the debate: "I didn't listen to my instincts in terms of preparing," Biden said.
Biden suggested that Trump's disruptions — from just a few feet away — had flustered him: "I realised that, even when I was answering a question and they turned his mic off, he was still shouting and I let it distract me. I'm not blaming it on that. But I realised that I just wasn't in control."
