The world No.63 gifted a spectator a towel between ends during the 6-3 6-4 win after she was struck on the forehead earlier in the opening set of the match in Washington.

The woman was sitting directly behind Paul when a return of serve from Kyrgios bounced over the wall at the back of the court and hit her.

Paul immediately checked on the spectator, who appeared shocked but otherwise unhurt, before Kyrgios enquired on her welfare from the other end of the court.

The Australian later presented her with one of the tournament's towels after switching ends, while again checking she was okay.

The 85-minute defeat of Paul continues Kyrgios's positive preparation for this month's US Open in New York.

Having skipped last week's singles event in Atlanta due to a knee complaint, the Wimbledon finalist continues to make a full recovery from his loss to Novak Djokovic as he looks to win the event he claimed in 2019.

Kyrgios set the tone with a forehand winner on the opening point of the match before breaking Paul's serve in the opening game.

In an otherwise confident display though, there were flashes of Kyrgios's trademark temper as he berated his box on occasion for a lack of support during key moments in the contest.

The most significant outburst came during the fifth game of the second set, Kyrgios lashing his supporters after sending a shot long to give Paul a break point.

"It's game point and you're just f***ing sitting there," Kyrgios yelled before holding serve to take a 3-2 lead.

Kyrgios will next face Reilly Opelka in the round of 16 after he defeated Denis Kudla 6-7 (8-6) 6-4 6-4.

The 27-year-old has faced world No.17 Opelka twice in his career, losing both encounters against the 211cm American.

Australian Associated Press