Olivia Fiechter is a surprise quarter-finalist at the US Open ©PSA

Olivia Fiechter upset fourth seed Amanda Sobhy in a back-and-forth, all-American third-round tie at the US Open squash tournament in Philadelphia.

The win means Fiechter is into the quarter-finals at a Professional Squash Association (PSA) World Tour Platinum event for the first time, and it is one of the biggest of her career.

It also represents Fiechter's first PSA World Tour victory over Sobhy.

Sobhy, who is up to third in the world rankings, twice came from a game down to level proceedings, but Fiechter was able to grind out an 11-1, 8-11, 11-3, 8-11, 11-9 win in 43 minutes.

And she will now fancy her chances of reaching the last four as Belgium's Nele Gilis - a fellow unseeded player - awaits in the quarter-finals.

Gilis beat the Canadian Hollie Naughton 11-7, 17-19, 12-10, 11-3.

Top seed Nour El Sherbini brushed aside fellow Egyptian Nadine Shahin 11-2, 11-6, 11-7 in 21 minutes, while reigning champion Nouran Gohar overcame Alison Waters 11-4, 11-8, 11-5.

Gohar will play a second English player in a row, Sarah-Jane Perry, in the semi-finals, with El Sherbini to face France's Melissa Alves.

Joelle King from New Zealand and Egypt's Hania El Hammamy will contest the other women's quarter-final.

In the men's draw, the seven remaining seeded players all safely advanced from round three.

Englishman James Willstrop - responsible for knocking out fifth seed Marwan ElShorbagy in the last round - this time saw off Mohamed Abouelghar in a topsy-turvy affair.

It took more than an hour, but Willstrop triumphed 11-9, 7-11, 9-11, 11-4, 11-3.

A third Egyptian opponent in a row, Tarek Momen, awaits in the last eight.

Top seed Ali Farag and defending champion Mohamed ElShorbagy both won all-Egyptian last-16 matches in four games to advance.