Coco Gauff admitted problems with her serve are “emotional” and “mental” after serving 19 doubles faults – including 11 in the final set – to crash out of the US Open.

The defending champion’s forehand and second serve fell apart as she made a total of 60 unforced errors to lose 6-3 4-6 6-3 to her Olympic team-mate Emma Navarro. ESPN analyst Chris Evert said: “Coco was clearly off. I have never seen her serve as badly as in this match.”

The defeat ended the world No.3’s 11-game winning streak in New York. And Gauff, 20, said: “I fought really hard today. Overall I think I played well for the most part. I just didn’t take care of my serve, so that was the biggest difference. Too many free points on my serve. Obviously I go down on my left side a lot on my serve, and it’s something I’m aware of, but it’s tough in the moment to I guess try not to do it. I was just trying to commit to it and just stay calm.”

Australian Open champion Aryna Sabalenka hired a mechanics coach to rebuild her serve two years ago before she made her Grand Slam breakthrough. And Gauff said she will consider doing the same to get over her own “mental hurdle”

The American said: “I definitely want to get other opinions because I think it’s sometimes more of an emotional, mental thing because if I go out on the practice court right now, I would make, like, 30 serves in a row. I’ve done it before.

“I think it’s also just kind of a mental hurdle that I have to get over when it comes with that. But I definitely want to look at other things because I don’t want to lose matches like this anymore.”

French Open semi-finalist Gauff also lost to Navarro in the fourth round at Wimbledon. She was a Team USA flag carrier at the Paris Olympics but went out in the last 16 and won only one match in her two hardcourt warm-up events in Toronto and Cincinnati. “I know this wasn’t a great couple of months,” she said.

Novak Djokovic and Carlos Alcaraz – the two male Olympic finalists – both went out in the first week and blamed the exertions of the Olympics.

But Gauff said: “I felt like physically and mentally ready, but for sure I think in Toronto and Cincy it was a little bit mentally draining, those tournaments having to switch surfaces. It’s tough when you’re trying to find your game. You’re just used to maybe a certain type of year.

“But coming into this, I didn’t feel any exhaustion. I was actually really excited to play. But that’s something you have to deal with every four years, so it’s not that big of a deal. Every Olympics I can qualify for and I’m healthy, I’m definitely going to play. So I don’t blame it on that.”

Navarro said: “This is the city I was born in so it is always pretty special playing here. Coco is a special player and I know she is going to come back and win here again here.”

She will next face Manhattan-born Spaniard Paula Badosa in her second consecutive Grand Slam semi-final. “I hope to have the American fans behind me,” said Navarro.