In the first meeting this century between two United States-born players this deep into a PSA World Series tournament, the 23-year-old came through in four games against world number 26 Blatchford.

After taking the first game 11-7 at the University Club of Chicago, Sobhy found herself drawn into a real battle as Blatchford upped her game to take the second 14-12 in a tie-break.

She could have gone behind in the third game, but she managed to get out of trouble and take it 12-10 before sealing the win with an 11-3 success in the fourth.

"It was difficult mentally out there against a good friend," said Sobhy, the highest-ranked US-born player ever.

"I got a bit edgy when things where getting close.

"She played well in the big points and I was really bummed to lose the second game.

"I finally felt like I relaxed in the fourth, so hopefully I can play the whole match tomorrow like that."

Sobhy will take on two-time defending champion Raneem El Welily of Egypt for a place in the semi-finals after the former world number one downed Hong Kong's Annie Au 11-8, 8-11, 11-4, 11-7.

Joining them in the last eight are 2014 tournament winner Laura Massaro of England, an 11-2, 11-8, 11-8 victor at the expense of Canada’s Samantha Cornett, and Egyptian world number four Nouran Gohar, who came from behind to defeat in-form Englishwoman Sarah-Jane Perry 6-11, 11-9, 11-9, 11-6.

In the men's draw, another in-form English player, James Willstrop, prevailed 11-4, 11-8, 8-11, 13-11 against Egypt’s Mohamed Abouelghar to advance through to the quarter-finals.

Willstrop thrashed Egypt’s Omar Mosaad 3-0 in round one and picked up from where he left off as he continued to hit accurate, precise lines and leave Abouelghar powerless to resist as he took a 2-0 lead.

Abouelghar took the third and was enjoying success in the fourth until 9-9, when Willstrop's beautifully deceptive cross-court shot set up match point - although he needed two more to convert the win.

Next up for Willstrop is 2015 world champion Gregory Gaultier after the Frenchman comfortably overcame England’s Daryl Selby 11-2, 11-0, 11-1.

The winner of that encounter will face either Spaniard Borja Golan, who beat Hong Kong’s Leo Au 1-11, 11-5, 11-4, 11-7 to reach his first PSA World Series quarter-final in over a year, or current world champion Karim Abdel Gawad of Egypt.

Gawad, the Qatar Classic and Tournament of Champions winner, beat compatriot Fares Dessouky 11-6, 10-12, 11-8, 11-9 in a hard-fought match.

Competition in Chicago is due to continue until March 1.