Clinton is beating Trump 42 per cent to 40 per cent among likely voters, according to a Morning Consult poll. She leads Trump among registered voters by 1 percentage point, 39 per cent to 38 per cent, Politico reported on Sunday.
Before her bout with pneumonia - and near collapse in public - Clinton was up 41-39.
The new poll suggested that Clinton might be winning back voters from third-party candidate Gary Johnson, whose support has declined from 10 per cent to 8 per cent.
Forty-three per cent of Republicans said that Trump was not the best pick to be the party's nominee, while 41 per cent of Democrats suggested the same of Clinton, a stunning level of regret in the electorate with 51 days until Election Day.
Half of Republicans said Trump was the party's best pick, while 55 per cent of Democrats said the same of Clinton, Politico noted.
According to the poll, over one-third of Clinton's supporters say they are voting against Trump, and 44 per cent of Trump's voters say they were voting against Clinton.
Still, 47 per cent of those polled think Clinton will win in November, and 33 per cent said Trump will. Last month, 53 per cent thought Clinton would be the next president.
The survey polled 1,639 likely voters and 1,861 registered voters online, on September 15 and 16.