American Mackenzie McDonald toppled former champion Kei Nishikori in three sets to claim a spot in the Citi Open final on Sunday against fifth seed Jannik Sinner.

McDonald triumphed over unseeded Japanese Nishikori 6-4 3-6 7-5 in two hours and 45 minutes in Washington DC.

The 26-year-old American's win qualifies him for his maiden ATP Tour final, sending down 4-1 aces while getting a slight edge with his return.

Nishikori, who triumphed in DC in 2015, fought back when trailing a set and a break in the second with four straight games to send it to a third.

The 31-year-old Japanese appeared to have the momentum but McDonald responded to outlast him.

McDonald will face 19-year-old Sinner who overcame another emerging talent, American 20-year-old Jenson Brooksby, 7-6 (7-2) 6-1.

Sinner becomes the first Italian to reach the final in DC and also progresses to his first ATP 500 decider.

The Italian continued his fine run in DC, having not yet dropped a set all tournament, marking a return to form after being on a four-match losing run.

Sinner saved three set points at 5-6 in the first to fight back to take the lead in a tiebreaker before a dominant second.