Nick Kyrgios overcame early back spasms to get the better of Russia's Daniil Medvedev in a pair of tiebreakers and win the Citi Open in Washington on Sunday.

The Australian won 7-6(6), 7-6(4) over a frustrated world number nine Medvedev for his sixth career ATP Tour victory. Kyrgios did not face a single break point and hit 18 aces, including two to close out the final. 

"It's great to get the win, but I proved to myself and a lot of people backing me that I still have it and can produce at the highest level," said Kyrgios, who defeated the tournament's number one seed Stefanos Tsitsipas in Saturday's semifinal.

"This week is probably one of the best weeks of my life in tennis," the 24-year-old said.

Kyrgios, mixing baseline rallies with drop shots, came from 4-1 down in the opening tiebreak to win it and took a medical timeout after the set, which seemed to help relieve the back spasms that had him stretching on the court during changeovers.

"I had some back spasms on the right side of my back," the Australian said. "I haven't played that much tennis in the last couple of months so this week the load has been pretty stressful."

The tension of the match no doubt added to the stress.

"We got very lucky to get out of the first set," Kyrgios said. "It was 5-2 and I got pretty lucky on some of those points. It could have easily been him standing in this position."

He still had trouble with Medvedev's serve, with the Russian winning 80 percent of his service points for the match.

Kyrgios eventually tied the second set at 6-6 and claimed the victory, his second of the year, in another tiebreak.

Before serving for the win, the Australian, as he had done twice previously in the match, went to a spectator to consult on where to serve.

Kyrgios then delivered a rocket for the win and fell on his back in relief.