Ben Shelton became the first American to win an ATP Masters 1000 title in Canada in more than two decades as he defeated Karen Khachanov 6-7 (5/7), 6-4, 7-6 (7/3) in Toronto final on Thursday.

The 22-year-old will rise one place to sixth in the world rankings, overtaking 24-time Grand Slam singles champion Novak Djokovic in the table.

"It's a surreal feeling," said Shelton, who is the first American since Andy Roddick in 2003 to win the Canadian trophy. "It's been a long week. It was not an easy path to the finals.

"But my best tennis just came out when it mattered the most."

The victory in two-and-three-quarter hours denied 11th seed Khachanov a second Masters title shied more than six years after he won his first at Paris Bercy in 2018.

Shelton, his right quadriceps muscle heavily strapped and at times appearing to be in some discomfort, stormed through a final set tiebreaker with leads of 3-0 and 5-2 on his opponent.

The left-hander lined up four match points and converted on the second to clinch the biggest title of his career.

The American fired 16 aces as he used his powerful serve to fuel the emotional comeback victory.

He had lost serve to fall behind 4-3 in the opening set, but pegged Khachanov back to level the set at 5-5 before Khachanov, seeded 11th, took the first set after saving three set points in the 12th game.

Khachanov sealed the 64-minute opener on his first chance with a leaping volley winner which Shelton could not touch.

For the second night in a row in a showpiece match, glitches in the heralded computerised line calling technology caused commotion.

The problem came to light in the fifth game of the second set, when the audio went out and left the players uncertain as to what balls were in and which were out.

After six minutes of discussion while play was halted, the system came back to life with Khachanov holding for 3-2 on the resumption as he scooped a volley winner past his opponent.

Shelton, loudly encouraged by his father-coach after almost every point, managed to break for a 5-4 lead and held the margin as he saved four break points in the next game before sending over a winner which left Khachanov on the ground trying to make the return.

With victory secured, Shelton scrambled into the stands to celebrate with his father, former ATP player Bryan Shelton. (AFP)