It comes as several key battleground states still have millions of mail-in ballots to count, with a record-breaking 102 million ballots cast early partly due to the Covid pandemic.
Donald Trump has made the unfounded claim the system is a "fraud on the American nation" and threatened to take the result to the Supreme Court a for decision.
It wouldn't be the first time a presidential election ended up in the highest court in America.
What happened in 2000?
Twenty years ago after the 7 November election day, the US found itself in a similar position on results day in the race between Democrat Al Gore and Republican George W Bush.
As the results flooded in, the two candidates were neck and neck in all but one of the country's 50 states.
It fell to Florida to decide.
Why was Florida at the centre of it all?
Known as the Sunshine State, Florida holds 29 highly coveted Electoral College votes.
To win, a presidential candidate needs to gain 270 Electoral College votes and different states have different numbers of them.
In 2000, US television networks initially called Florida in Al Gore's favour, but later returned its status to "too close to call" before finally declaring victory for George W Bush.
What followed was multiple recounts and a legal fight that ended up in the Supreme Court.
What happened in Florida to confuse the result?
Much of the controversy and confusion lay in the punch card voting system used in the state.
The ambiguity of the result was blamed on the "hanging chads" - what Americans called voting cards that had not been neatly punched through.
Hanging chads led to the card not being counted by the tabulating machines calculating the votes.
That was not the only issue with them.
Polling cards that had a clear indentation but no hole were dubbed "pregnant chads," while "swinging chads" were still attached at two corners, also leading to the votes being discarded.
How was the election resolved?
Al Gore initially sought a recount of a small sample of Florida's disputed ballots and it is estimated 2.9% of votes cast never made it into the certified totals.
The Democrat's lawyers then launched a bid for a recount of all 1.8 million ballots cast in four predominantly Democratic counties - before calling for further recounts elsewhere.
The Florida Supreme Court ordered a state-wide recount of "undervotes" – a ballot that does not clearly indicate the voter's preference – on December 8, a month after Election Day.
It was seemingly a huge victory for Mr Gore, but it was stopped the next day by the US Supreme Court and the legal action returned to state courts.
On December 13, the closest presidential contest in decades finally ended.
Following a 5-4 vote in the Supreme Court, 36 days after Americans cast their ballots, judges ruled George W Bush's victory did stand.
Mr Gore accepted the verdict and with that Mr Bush was on his way to the White House.
In Florida, Mr Bush had officially won by just 537 ballots in the state on a turnout of 6.1 million.