Hillary Clinton returned on Thursday to the campaign fray in a tightening race against Republican Donald Trump, who released new details of his physical fitness in response to the health scare that sidelined his rival.

Seeking to turn the page after her poorly handled bout with pneumonia, Clinton was headed to North Carolina and then the US capital Washington as she resumed the White House race after a three-day convalescence.

"Welcome back to ‘Stronger Together,’" the Democratic nominee quipped cheerily to reporters on board her campaign plane.

Asked how she was feeling, the 68-year-old former secretary of state replied: "I am doing great, thank you so much!"

Clinton was taken ill Sunday during a 9/11 memorial ceremony in New York where she was seen stumbling limp-legged into her vehicle, an episode that raised tough questions about her campaign’s transparency.

With the candidates’ health suddenly at the forefront of the campaign, Clinton looked to head off further scrutiny by releasing new medical records yesterday indicating that she was "fit to serve" as president.

The disclosure came as the media-savvy Trump, 70, teased new health data of his own during the taping of a medical chat show, before publishing it today in full.

The one-page letter from his long-time doctor lists various lab results, including for cholesterol, blood pressure and liver and thyroid function — all deemed to be within the normal range.

While Trump is shown to be overweight, with a body mass index of 29.5, his doctor Harold Bornstein declared the Republican nominee to be "in excellent physical health." 

Trump had made a point of refraining from harsh attacks on his convalescent rival — but the candidates were quick to resume their jousting.

Team Trump included a veiled jab at Clinton in a statement accompanying his health update: "We are pleased to disclose all of the test results which show that Trump is in excellent health, and has the stamina to endure — uninterrupted — the rigors of a punishing and unprecedented presidential campaign."

While addressing the Economic Club of New York, Trump slammed the policies of Clinton and President Barack Obama as having doubled the national debt as he promised his presidency would bring about "an American economic revival."

Clinton, meanwhile, slammed the brash billionaire, who has repeatedly praised Russian President Vladimir Putin, for his "alarming closeness with the Kremlin."

"Donald Trump has run a deplorable campaign," she told the Tom Joyner radio show.