The Mexican currency's response was a sharp turnaround as investors have punished the peso in recent weeks. The greenback had climbed nearly 10 per cent since early September as polls started tightening, extending a rout for the Mexican currency which has lost nearly a third of its value over the past two years. Net short positions on the peso jumped 37 per cent in the week ending September 20 and were at the highest in more than 20 years of data.

"On balance, the markets perceive that the first debate went to Mrs Clinton, hence the US dollar is under a bit of pressure," said Annette Beacher, chief Asia-Pacific macro strategist at TD Securities. "Risk assets are a little firmer after a soft overnight session."

The Australian dollar also rose 0.6 per cent to US76.63¢ during the debate.

The Japanese yen, a safe haven currency for foreign exchange traders, weakened as relief swept through financial markets. Japan's currency had climbed earlier as a Bloomberg Politics poll showed the candidates deadlocked before the debate.

The yen dropped 0.4 per cent to 100.74 per dollar during the 90-minute debate. It had touched 100.09, the strongest since August 26 before the start of the debate.

"The market has been ignoring the US election as a risk just as it had with Brexit," said Peter Rosenstreich, head of market strategy of Swissquote Bank. "But the combination of new elections polls and uncertainty of tonight's debate has put the market on edge."

In other markets, S & P 500 Index futures rallied 0.6 per cent, after declining by as much as 0.3 per cent.

The US benchmark has risen by an average of 0.85 per cent on the day following the first debate in the run-up to a general election, according to data going back to 1992 compiled by Strategas Research Partners.


Opinion polls have swung by 3 per cent on average, within most surveys' margin of error, following the first debates of election campaigns since 1984, Strategas noted.

Gold as a safe haven asset slid 0.4 per cent during the debate.

"Gold appears to be the first choice for those seeking refuge from the impending uncertainty as we kick off the presidential debates," said Gary Huxtable, strategist at Atlantic Pacific Securities.

With Bloomberg