Arizona Democratic Senate candidate Kyrsten Sinema suddenly took a narrow 9,600-vote lead over GOP opponent Martha McSally late Thursday, with some 400,000 votes left to count in a closely-watched race that will determine the size and influence of the Republican Senate majority in January.

The whirlwind reversal in the race to replace retiring Republican Sen. Jeff Flake was a striking reminder that Election Day, though it concluded on Tuesday, might not be out of surprises. McSally had consistently led Sinema since Election Day, and was ahead by 17,000 votes as of early Thursday.

But Sinema pulled ahead by just 2,000 votes that afternoon, and her lead expanded by approximately another 7,000 votes the next hour.

Sinema now has 932,870 votes statewide, while McSally has 923,260 and Green Party candidate Angela Green currently has 43,838, according to results provided by election officials at 8 p.m. EST.

Some 345,000 votes are yet to be counted in Maricopa County alone, including a bevy of mail-in ballots dropped off at polling places on Election Day. Republicans expect those votes to lean for McSally.

As the tense process proceeded Thursday, McSally, who previously flew close air support combat missions in an A-10 warplane above Iraq and Kuwait, lightened the mood by tweeting a picture of herself at the dentist's office.

"With half a million ballots left to count, we remain confident that as votes continue to come in from counties across the state, Martha McSally will be elected Arizona's next Senator," McSally campaign head Jim Bognet said.

Still, Republican Gov. Doug Ducey, who easily cruised to reelection on Tuesday, was significantly outperforming McSally in several GOP-leaning areas of Arizona. Some analysts, including Fox News host Laura Ingraham, said it's very unlikely McSally will reclaim the lead.

Though she has only a little more than 2 percent of the overall vote, Green is playing an outsized role in the proceedings. The Green Party candidate, who dropped out of the race before Election Day and threw her support behind Sinema, is facing accusations of playing a potential "spoiler" role in the contest.

APPROPRIATELY-NAMED GREEN PARTY CANDIDATE ANGELA GREEN DENIES THAT SHE PLAYED SPOILER IN ARIZONA SENATE RACE

“I know the Democrats think all 3rd party voters would’ve chosen them, but I’m sorry to say that’s just not the case. Same goes for the Republicans,” Green wrote on her campaign website. “No 3rd party candidate should have to endure accusations of being 'spoilers' just because the 'winner takes all' two-party system is severely broken. This is a complete travesty. It’s time for change.”

She added: “I knew I wasn’t going to win, so being a true candidate for the people and not the politics, I felt that if I withdrew and could endorse a candidate closest to the ideas and views of those whom I represent, then at least I can feel as though this withdrawal from the Senate race will not be in vain.”

Arizona GOP officials sent mailers to Democratic voters during the campaign that highlighted Green's liberal views and tied her to Vermont Sen. Bernie Sanders, in a roundabout bid to convince them to support Green instead of Sinema.

Fox News' Hillary Vaughn and Andrew O'Reilly contributed to this report.