Washington, Sep 16 (IANS): Democratic presidential nominee Hillary Clinton has vowed that she will never give up as she hit the campaign trail again following a three-day rest after being diagnosed with pneumonia.

Speaking at a rally in greensboro, North Carolina, Clinton said her time off was a gift, allowing her to reflect on the campaign, BBC reported.

"People accuse me of all kinds of things," she said, "but nobody ever accuses me of quitting and I will never give up, I'll never walk away, no matter how tough the going gets," she told her supporters at Thursday's rally.

Polls indicate a tightening White House race, with 54 days to the election day -- November 8.

The former State Secretary's return comes a day after her doctor said she was "healthy and fit".

"With just two months to go until the election day, sitting at home was pretty much the last place I want to be," Clinton said.

The 68-year-old said she felt "lucky" that she could afford to take a few days off, compared with the millions of Americans who could not.

At a brief press conference afterwards, when Clinton was asked whether she had shared details of her pneumonia diagnosis with her running mate, Tim Kaine, the Democrat replied that "many senior staff knew and information was provided to a number of people".

"This was an ailment that many people just power through," she continued, "and that's what I thought I would do as well."

She then jetted off to a give a speech at an event in Washington DC.

The race has focused on both candidates' state of health and medical records in recent days.

As countless thousands of armchair physicians looked on, Clinton took to the campaign trail for the first time since her near-collapse on Sunday, in a ceremony marking the 15th anniversary of the 9/11 attacks.

She said one of her strengths was never quitting. Early on, she pivoted from talking about her illness to the struggles of Americans who cannot afford quality healthcare.

Although Clinton took swipes at Republican presidential nominee Donald Trump, she only referred to him as "my opponent".

She said she wants to give Americans "something to vote for, not just against".

According to EFE news, Clinton said of Trump that she knows she will "never be the showman my opponent is, and that's OK with me."

"Let's talk about what really matters," she said. "And here's my promise to you. I'm going to close my campaign the way I began my career and the way I will serve as your president -- focused on opportunities for kids and fairness for families. From now until November 8, everywhere I go I'm going to talk about my ideas for our country."

On Thursday, Trump released a letter from his doctor saying he is in "excellent physical health", albeit overweight.

He earlier this year released a brief memo from the same physician asserting he would be "the healthiest individual ever elected to the presidency".

At a rally in Ohio on Wednesday, Trump used Clinton's recent health scare to cast doubt on her stamina, saying: "I don't know folks, do you think Clinton would be able to stand up here for an hour?"

The first debate between Clinton and Trump will take place on September 26 at Hofstra University on Long Island, followed by one in St. Louis on October 9 and a final one on October 19 in Las Vegas.