Kei Nishikori secured his 10th ATP World Tour title with a hard-fought win over John Isner in the Citi Open final.

The Japanese star battled from behind to edge Isner 4-6 6-4 6-4 in the final of the ATP 500 event played on hard-courts in Washington.

Isner sent down 18 aces, but he was unable to repeat the win he claimed over Nishikori in the quarter-finals in Miami in March.

It marked Nishikori's third ATP title of the year and first since April, while Isner's eight-match winning run comes to an end.

"It was a wonderful week," Nishikori said in an on-court interview, having struck 24 winners and just 10 unforced errors in his win.

"I felt really comfortable playing here and it was a very fun week."

Isner won fewer return points than Nishikori in the first set, but he broke to 15 in the 10th game to win the set.

Nishikori responded by breaking in the opening game of the second set, and that would be all he needed as Isner struggled to make inroads against serve.

That trend continued in the deciding set as Nishikori, serving 70 per cent of first serves in, dominated.

He lost just three points on serve and a break in the third game sent him on his way to victory.