Sinema's lead grew to 29,832 votes as of 7 p.m. and gave her a 1.4 percentage point advantage.
Overall, Sinema has 1,048,655 votes and McSally has 1,018,823. Green Party candidate Angela Green has 49,584.
The Arizona Republic estimates there are 260,000 ballots still to be counted in the state. The number for Maricopa County is 198,000, where, so far, Sinema has been winning by 3.5 percentage points.
While McSally's vote deficit grew, Republicans took aim at the election process. In a statement Saturday, the National Republican Senatorial Committee attacked Maricopa County Recorder Adrian Fontes, a Democrat, saying he "has been using his position to cook the books for Kyrsten Sinema."