United States President Joe Biden on Friday attempted to defend his candidacy for the country’s upcoming presidential elections in November a day after a disappointing debate against his rival Donald Trump.

According to AFP, Biden, 81, appeared at a campaign rally in North Carolina to deliver a fiery speech amid concerns about his replacement as the Democratic Party’s nominee and a call from the nation’s most influential newspaper to step aside.

"I don't walk as easy as I used to. I don't speak as smoothly as I used to. I don't debate as well as I used to," Biden admitted to supporters in unusually confessional remarks.

"But I know how to tell the truth. I know how to do this job," he said to huge cheers, vowing "when you get knocked down, you get back up". 

Biden's team was in damage-control mode after Thursday's debate when he often hesitated, tripped over words and lost his train of thought, exacerbating fears about his ability to serve another term.

The US president attempted to address concerns about his advanced age and Trump's alleged lies, but failed to counter his bombastic rival's unchallenged false or misleading statements on various issues, including the economy and immigration, according to AFP.

On Friday, Biden delivered the lines Democrats wished they had heard in the televised debate.

"Did you see Trump last night? My guess is he set — and I mean this sincerely — a new record for the most lies told in a single debate," Biden said.

Trump also returned to the campaign trail Friday, speaking at a rally in Virginia and launching his familiar attacks on Biden in a rambling speech.

"It's not his age, it's his competence," Trump said. "The question every voter should be asking themselves today is not whether Joe Biden can survive a 90-minute debate performance, but whether America can survive four more years of crooked Joe Biden."