Shelton only turned professional in August last year but he reached the quarter-finals of the Australian Open in January and the semi-finals of the US Open last month.

The 21-year-old had to battle through a series of three-set matches to reach the Tokyo final but he made short work of world number 50 Karatsev, winning in just over 1hr 20min.

Shelton won the first set after breaking his opponent for a 6-5 lead and holding his serve in the next game.

Karatsev struggled to regain his composure in the second set and smashed his racquet in frustration as unforced errors began to creep into his game.

Karatsev hit a shot long on match point to hand Shelton the victory.

The American celebrated by running to his team and hugging his father, Bryan, who is also his coach.

Karatsev had reached the final winning all his matches in straight sets, including victories over fourth seed Alex de Minaur and sixth seed Frances Tiafoe.

Shelton also beat Karatsev at the US Open.