Donald Trump at a rally on Sunday had rare praise for Hillary Clinton.
Trump told the New Hampshire crowd that Clinton had more energy than Biden.
He also described her as more intelligent than the current Democrat leader.
Clinton has said in a new podcast a second Trump term would make her feel ill.
She said that his re-election would leave her 'literally sick to my stomach'.
Donald Trump has described Hillary Clinton as having 'more energy' and being 'a more intelligent person' than his current Democrat rival Joe Biden, in a rare moment of praise for his former challenger.
Trump, 74, has had little positive to say about Clinton, 72, in the years since their 2016 election battle.
But on Sunday the president gave Clinton credit for her intelligence - albeit in comparison with Biden, who he endlessly mocks as senile and mentally deficient.
Addressing a crowd in Londonderry, New Hampshire - the day after events in four states in one day - Trump asked: 'You think Sleepy Joe would be doing these things? I don't think so.'
He continued: 'He'll go back to bed. Hillary used to spend a lot of time in bed too. But she had more energy than him. She did.'
Trump, who famously believes that exercising means you expend energy and feel constantly tired, has always prided himself on his own stamina and strength.
He has repeatedly belittled Biden for respecting CDC guidelines and avoiding large rallies, saying it was a sign of Biden's age and poor health.
On Sunday, he added: 'They said: "What's the difference between Crooked Hillary Clinton and Biden?"
'I said: "Well, obviously they're both crooked," but let's get out of that.
'The big difference is, she had more energy — but not much — and she's a more intelligent person. I will say that.
'She's a much more intelligent person. Until she used the word "deplorables," I thought she was very smart.'
Clinton famously called his supporters a 'basket of deplorables' during the 2016 campaign.
'You know, to just be grossly generalistic, you could put half of Trump's supporters into what I call the 'basket of deplorables,' Clinton said, during an LGBT fundraiser in Manhattan in September 2016.
'The racist, sexist, homophobic, xenophobic, Islamophobic — you name it.'
In a new podcast, to be aired on Monday, Clinton says that she would be 'sick to my stomach' if Trump was re-elected.
'It makes me literally sick to my stomach to think that we'd have four more years of this abuse and destruction of our institutions,' she said, according to Axios, which obtained the episode.
'And damaging of our norms and our values. And lessening of our leadership. And the list goes on.'
Clinton said she was sure many Republicans felt the same way.
'Most Republicans are going to want to close the page.
'They want to see him gone as much as we do, but they can't say it publicly.'
She said the Republicans 'have been cowards, spineless enablers'.