Results from Nevada won't be known until Thursday morning but Joe Biden is expected to squeak a narrow victory.
The state was won by Hillary Clinton in 2016 and it is expected that the majority of around 400,000 outstanding mail-in votes will go for Biden.
Nevada officials originally hoped to release results not long after polls closed, but they were delayed until 9am on Thursday.
The Trump campaign said on Thursday that they would file a lawsuit to question the vote count and try to halt it.
A press conference is expected later.
Biden has a lead of just 0.64% or 7,647 votes with 588,252 ballots compared to Trump's 580,605
The state said it had counted all votes cast in-person either on election day or beforehand and all mail-in ballots received before election day. However it still needs to count postal votes received on 4 November, while it will also accept mail-in ballots arriving as late as next Tuesday, as long as they were posted before the polls closed.
It means the full number of votes won by Donald Trump and Joe Biden might not be fully known until next Tuesday, although close to 1.2million votes had been counted as of Wednesday morning.
Nevada's secretary of state's office said it was 'difficult to estimate' how many ballots were left to count as it sent every voter in the state a ballot in the post, even if people subsequently chose to vote in person.
But while Biden has a slim lead of fewer than 8,000 votes as of the latest count, around 75,000 mail ballots could still need to be counted in Clark County, home to Las Vegas and close to three-quarters of the state's population.
Polling workers in North Las Vegas, Nevada, after the polls closed on election day yesterday
People including Diane Stein (center L) and Marietta Navarro (center R) vote at Desert Breeze Community Center on November 3, 2020 in Las Vegas
Biden is currently winning this part of the state by 422,762 votes to 362,573, and Jon Ralston, editor of The Nevada Independent, said the 337,000 previously counted absentee ballots from the county 'have dramatically favoured the Democrats so far by more than two to one', potentially giving the Democrat some breathing space.
Meanwhile the Trump campaign also lost a last-minute bid to try and stop mail ballots in Clark Country from being processed using machines which automatically verify signatures, after their claim that the process was opaque was thrown out by the state's Supreme Court on election day.
Initial results in the state were also likely delayed by the fact 30 polling spots were allowed to stay open for an extra hour following a court order, after they faced 'technical problems'. No results are allowed to be posted until all ballots have been cast in Nevada.