Democratic presidential nominee Hillary Clinton went for the heart of opponent Donald Trump's candidacy - his claim that being a businessman would make him a good president - and her attacks paid off, said political observers after the first one-on-one debate between the candidates, held less than two months before the Nov 8 election.

"His claim that he is a great businessman has fuelled his candidacy from Day One," wrote Dr Elaine Kamarck, a senior fellow in the governance studies programme of The Brookings Institution.

"If Hillary can take that away from him, she will be able to hold on to her lead, sway remaining undecideds, and perhaps even erode some of his support."

Mrs Clinton criticised her Republican opponent's economic plan, calling it "trumped up, trickle down" economics, trying to paint him as a supporter of those already at the top, then went on to describe him as a bully who had "stiffed over" thousands of people in the course of his business career.

She also made it clear that he could be hiding many things by not releasing his tax returns, including the possibility that he had not paid federal taxes.

Noting that Mr Trump could not engage in policy debate because he is "proudly ignorant of basic policy", Dr Kamarck added: "This is the best strategy the Democrats have left."

Not only was the attack successful, experts noted, Mr Trump also took the bait almost every time.

"She threw a lot of accusations out at him and, instead of ignoring it, he went after it," said assistant director of debate at the University of Miami Patrick Waldinger. "He needs to know when it is appropriate to respond and figure out when it is okay to leave those issues behind. He doesn't need to respond to everything, especially when it reinforces a negative."

Where Mr Trump rambled on, his Democratic rival kept her response to attacks concise, especially when it came to her private e-mail server.

"She simply said it was a mistake and stopped talking. She looked good, she smiled a lot. She was smart but not wonky," Dr Kamarck said.

Many experts said Mr Trump's sub-par performance should make him rethink his debate preparation strategy going forward.

While Mrs Clinton held mock debates with an aide posing as the billionaire businessman, Mr Trump favoured informal sessions with his team, who simply gave him advice.

"A more formalised preparation structure would serve him well," said University of Michigan's director of debate Aaron Kall. "She got better as the night went on, while Trump looked exasperated and crawled slowly to the finish line."

But Mr Kall added that, while Mrs Clinton's debate performance was unlikely to "alter the trajectory of the race", it would "provide a much-needed boost of momentum and positively impact the media's coverage of the campaign".

He noted: "In such a close race, even small movements at the margins could be a significant asset at this stage."

Even international markets seemed to declare Mrs Clinton the winner. The Mexican peso, in particular, surged more than 2 per cent after hitting record lows in recent days over concerns that Mr Trump might be inching to the presidency - a situation that might threaten Mexico's exports to the United States.

The rise and fall of the peso have been tied closely to Mr Trump's polling numbers in recent weeks, as Mexico remains wary of possible trade protectionist measures he might adopt, coupled with his hardline immigration stance to "build a wall".

While markets have taken to Mrs Clinton as the candidate offering stability, Mr Trump is seen as a candidate who would mean an uncertain future for the US economy and trade deals.

Citigroup's experts have said that an election victory for Mr Trump could sink equities and might also spur volatility in gold and currency markets, according to Bloomberg.

Asian markets, too, perceived that Mrs Clinton had outperformed her opponent. The Hang Seng China Enterprises Index of mainland shares traded in Hong Kong and Vietnam's VN Index both climbed 1.2 per cent, while South Korea's Kospi index advanced 0.8 per cent.

"Markets started to call the debate for Hillary within the first 15 minutes or so, with the Mexican peso surging in what is probably the busiest Asian session in years," senior currency analyst Sean Callow at Westpac in Sydney told Reuters.