Italian teenager Jannik Sinner has claimed his third career ATP Tour title with a three-set victory over Mackenzie McDonald in the Citi Open final on Sunday.

The sixth seed triumphed in two hours and 52 minutes, winning 7-5 4-6 7-5 over the unseeded American, who was competing in his first ATP Tour final.

The Washington, D.C. victory was 19-year-old Sinner's first ATP 500 title and his third on tour, having triumphed in Sofia late last year and Melbourne earlier this year.

Home favourite McDonald put plenty of pressure on Sinner, forcing the Italian to save seven break points for the match.

McDonald also staved off 16 break points in an enthralling contest where the Italian sent down nine aces to the American's five.

Sinner lost his serve twice in the opening set and needed 11 set points but still won 7-5, before the American hit back, dropping further behind the baseline to good effect in the second, levelling it with a 6-4 win.

The Italian claimed an early break in the last to open up a 3-0 lead, before failing to convert two championship points at 5-2.

McDonald brought it back to 5-5, with numerous dramatic rallies throughout, before the 2019 Next Gen ATP Finals champion broke in the 12th game to triumph in the third set.