Rep. Kyrsten Sinema (D) took the lead in Arizona's too-close-to-call Senate race on Thursday night, holding a thin lead of 9,610 votes over Rep. Martha McSally (R).

Sinema holds a 0.5-point lead with votes rolling in, predominantly from Maricopa County, according to new numbers from the Arizona secretary of state's website. Votes were also posted from Pinal and Gila counties as well as Pima County, a Democratic-leaning county that's home to the city Tucson.

The tally marks a shift from McSally's initial 1-point lead as of earlier on Thursday, when the GOP congresswoman was up by 17,073 votes.

An Arizona source told The Hill that the latest numbers came from roughly 150,000 votes from Maricopa County, with another 345,000 outstanding votes still left from that county, which includes the capital of Phoenix, as well as Sinema's own congressional district.

As Sinema pulled slightly ahead on Thursday, a judge rejected GOP efforts to challenge the state's mail-in ballot counting procedures, according to The Associated Press.

Judge Margaret Mahoney set a hearing on the GOP lawsuit for Friday regarding about 5,600 votes from Maricopa County, which has been steadily releasing numbers on mail-in ballots since Tuesday's election.

Some county recorders have been calling voters who submitted mail-in ballots with signatures that don't match what's on file to verify their signature. Republicans allege in the suit that this procedure violates state law.

But Mahoney on Thursday said it was too soon to tell those counties to stop contacting those voters. The judge also wouldn't tell the counties to temporarily separate mail ballots verified through this procedure.

A total of approximately 500,000 votes remain uncounted across the state.

The Maricopa County recorder announced that the next update will be posted on Friday at 5 p.m. local time.

Sinema and McSally are locked in one of the fiercest Senate battles this cycle in the race to replace retiring Sen. Jeff Flake (R-Ariz.), an outspoken critic of President Trump.

Arizona was one of the few states this cycle where Democrats had an opportunity to flip a seat in their uphill fight for the Senate majority. While it's traditionally a red state, Trump carried Arizona by less than 5 points in 2016, which is a closer margin than previous GOP presidential nominees. And Democrats haven't won a Senate seat in Arizona since 1988.

Prior to the first batch of outstanding votes being released, Sinema's campaign released a statement voicing confidence about her prospects when all the remaining ballots are tallied.

"Arizonans must have faith that their votes are counted, and we are working diligently to ensure that count proceeds in a fair, transparent, and timely manner that voters can trust," Sinema campaign manager Andrew Piatt said, adding that the campaign believes she'll be elected after the remaining votes are counted.

McSally's campaign projected similar confidence in a Thursday night statement.

"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 CEO Jim Bognet said.

Arizona isn't the only Senate race that's left to be called. In Florida, Sen. Bill Nelson (D-Fla.) and his Republican opponent Rick Scott are also locked in a close race that looks headed to a recount.

And Mississippi's special Senate election is headed to a Nov. 27 runoff, with neither Sen. Cindy Hyde-Smith (R-Miss.) or former Agriculture Sec. Mike Espy (D) able to clear the 50-percent threshold needed to avoid one. Hyde-Smith is highly favored to win the runoff in the ruby-red state.

Regardless of the outstanding races, Republicans will hang onto their Senate majority after flipping seats in Missouri, Indiana and North Dakota. But depending on the outcome of those races, Republicans could still be governing with a small majority.

The current Senate breakdown is 51 seats for Republicans, and 46 for Democrats.

-Updated at 10:31 p.m.