Two-time champion Naomi Osaka is out of the US Open after suffering a stunning defeat in the third round to teenager Leylah Fernandez.

The 18-year-old Canadian won 5-7 7-6 (6-2) 6-4 to end Osaka’s title defence on Friday.

Up a set and 6-5, Osaka had the chance to close out the third-round contest before the four-time major champion suffered an epic meltdown on Arthur Ashe Stadium, where fans booed the titleholder.

Osaka, who is in the spotlight amid her mental health concerns after withdrawing from the French Open and skipping Wimbledon, threw her racquet three times in the unsuccessful second-set tie-break.

The Japanese star received a warning after hitting a ball into the crowd in the final set and while she tried to dig herself out of a hole, Fernandez completed the boilover in New York.

There were no breaks of serve through nine games – Osaka serving to stay in the set at 5-4 and did so easily before flicking the switch.

Former world No.1 Osaka broke to love in the very next game, reeling off nine successive points to take the opening set.

In the third round of the US Open for the first time and with a 1-2 record against top-10 players, Fernandez was not overawed on the big stage.

Behind her big first serve and powerful baseline hitting, the youngster ensured the second set followed a similar pattern as Osaka was prevented from racing away with the match.

It appeared as though the second set would go down the same path as the first after Osaka broke for a 6-5 lead.

But Osaka imploded on centre court after failing to serve out the match, she lost her composure in the tie-break, throwing her racquet on numerous occasions as Fernandez forced a deciding set with ease.

Osaka immediately left the court and emerged with a towel over her head before the start of the final set, however, she still looked off her game and was broken in the opener.

Continuing to struggle, Osaka was then given a warning for hitting the ball into the crowd, though she boosted her confidence by holding serve and avoiding falling 3-0 behind.

But Fernandez’s sole break point was all she needed, sending the defending champion home to the delight of the crowd.

Earlier, Simona Halep battled to victory over Elena Rybakina while Garbine Muguruza also enjoyed success against last year’s finalist Victoria Azarenka.

Playing with heavy strapping on her right thigh, the injury-riddled Romanian saved four set points in a remarkable first-set tie-break and finished strongly for a 7-6 (13-11) 4-6 6-3 victory.

Halep, who was down an early break in all three sets, said: “Before the match, I was super stressed and I told (Australian coach) Darren (Cahill) that my forehand is lost. So I cannot feel it anymore.

“I had emotions. I was nervous. And I was crazy a little bit on court, as well, the frustrations got me a little bit. But I fought, and this victory gives me a lot of confidence that my game is coming back and also the fighting spirit is there.”

Ninth seed Muguruza proved too strong for Azarenka, winning 6-4 3-6 6-2 to set up another blockbuster clash against French Open champion Barbora Krejcikova.

Krejcikova, whose rise has been so rapid that this is her main draw debut in New York, was a comfortable 6-4 6-2 winner over Kamilla Rakhimova.

German Angelique Kerber triumphed in the battle of the former champions, downing Sloane Stephens 5-7 6-2 6-3.

-AAP