NEW YORK - Naomi Osaka became the first Japanese to win a Grand Slam singles title on Saturday as she shocked former world No 1 Serena Williams of the United States 6-2, 6-4 in the final to win the 2018 US Open.

Playing her first Grand Slam final, the 20-year-old and No 20 seed beat idol Serena for the second time in 2018, backing up her Miami Open stunner with a 79-minute victory at Arthur Ashe Stadium.

The final was marred by Williams' second set outburst after chair umpire Carlos Ramos warned her for receiving coaching in the second game.

Williams was then handed a point penalty in the sixth game after breaking a racquet, which was her second code violation.

Williams continued to argue with Ramos and even yelled at the umpire from Portugal.

"You owe me an apology," Williams said. "I have never cheated in my life!"

"You're attacking my character," she said. "You will never, ever be on another court of mine. You are the liar!"

Ramos finally handed the 23-time Grand Slam winner a game penalty that put Osaka one game from victory at 5-3 in the second set.

Although Williams won the next game, Osaka stayed focused to hold serve for the historic victory.