The former world No 1 from Pontefract had already pulled a memorable display out of the bag to defeat world No 3 Omar Mosaad in the first round and had to call upon all his experience and energy to progress further after a 70-minute encounter with world No 20 Mohamed Abouelghar.
After losing the opening game at Drexel University, the unseeded Willstrop steadied himself and countered everything that Abouelghar threw at him to keep his tournament hopes intact, the 33-year-old Yorkshireman eventually prevailing 9-11, 11-9, 11-6, 5-11, 11-7
“That was an hour and a bit of frantic intensity,” said Willstrop.
“Hats off to him, he made me work so hard.
“I couldn’t get the ball off his racket in the middle area and he was playing excellent shots across me and that got me on edge. I had to play very, very well to beat him.
“I’m just so pleased to get through. I want to keep on playing - I don’t want to stop. I feel in a bit of a groove so I was so pleased to get through at the end and hopefully I can keep progressing.”
Willstrop will now go up against South African Stephen Coppinger for a place in the semi-finals after he came out on top in his second round match with Malaysian qualifier Nafiizwan Adnan.
Elsewhere, Harrogate’s Chris Simpson pushed world No 1 and top seed Mohamed Elshorbagy close before losing out 10-12, 11-9, 12-10, 11-5 after just over an hour of frenetic play.
Sheffield’s Nick Matthew - seeded fourth - will take on Germany’s Simon Rosner on Tuesday night.
Results – Men’s 2016 Delaware Investments U.S. Open: Second Round - Top Half
 Mohamed ElShorbagy (EGY) bt Chris Simpson (ENG) 3-1: 10-12, 11-9, 12-10, 11-5 (64m)
 Marwan ElShorbagy (EGY) bt [Q] Diego Elias (PER) 3-1: 3-11, 11-4, 11-4, 11-6 (48m)
Stephen Coppinger (RSA) bt [Q] Nafiizwan Adnan (MAS) 3-2: 11-7, 5-11, 9-11, 11-4, 11-9 (82m)
James Willstrop (ENG) bt Mohamed Abouelghar (EGY) 3-2: 9-11, 11-9, 11-6, 5-11, 11-7 (70m)