The 28-year-old from New York, winner of a record eighth successive US Nationals' title earlier in the month, faced Canadian qualifier Shawn Delierre. The Montreal-based 29-year-old is ranked 54 in the world, 26 places below Illingworth, but boasted a 3/2 winning record over his North American rival going into the match - and beat Illingworth in straight games in their last meeting in the 2011 US Open.
Delierre took a 2/1 lead, winning the third game in a tie-break, and moved 8-4 ahead in the fourth. But Illingworth recovered to draw level after another tie-break game - and again came from behind in the decider before clinching his 11-2, 9-11, 10-12, 12-10, 11-8 victory.
Illingworth will now face second-seeded Dutchman Laurens Jan Anjema, the world No9 who faced Australian Cameron Pilley - again a player who boasted a winning Tour record over his higher-ranked opponent.
But Dutch champion Anjema reduced the deficit by beating world No15 Pilley - albeit in 63 minutes - 11-7, 12-10, 7-11, 11-8 to claim his place in the last eight.
Frenchman Thierry Lincou, the highest-ranked unseeded player in the draw, eased into the quarter-finals after beating New Zealand's Martin Knight 11-2, 11-9, 11-4.
In a match befitting a final in most PSA Tour event, the 2010 Bluenose champion will now face fellow former world number one and world champion Amr Shabana, the top seed from Egypt making his debut in Halifax.
The charismatic left-hander got off to sluggish start against qualifier Chris Simpson as the Englishman quickly established a 6-1 lead. But Shabana soon settled into the match and took just 33 minutes to beat the Guernsey-born 24-year-old 11-9, 12-10, 11-4.
Shabana and Lincou, who first met on the PSA Tour in September 2001, will celebrate their 23rd clash on the all-glass court at the Rebecca Cohn Auditorium - with the Egyptian currently boasting a 13-9 head-to-head lead.
RESULTS: PSA International 50 Bluenose Classic, Halifax, Nova Scotia, Canada
 Amr Shabana (EGY) bt [Q] Chris Simpson (ENG) 11-9, 12-10, 11-4 (33m)
Thierry Lincou (FRA) bt Martin Knight (NZL) 11-2, 11-9, 11-4 (30m)
Stephen Coppinger (RSA) bt Andrew Schnell (CAN) 11-6, 11-8, 11-2 (31m)
 Hisham Mohd Ashour (EGY) bt Shahier Razik (CAN) 11-3, 11-4, 2-0 ret. (20m)
 Daryl Selby (ENG) bt [Q] Cesar Salazar (MEX) 11-6, 11-3, 11-8 (40m)
Borja Golan (ESP) bt [Q] Jan Koukal (CZE) 11-5, 11-4, 11-7 (42m)
Julian Illingworth (USA) bt [Q] Shawn Delierre (CAN) 11-2, 9-11, 10-12, 12-10, 11-8 (69m)
 Laurens Jan Anjema (NED) bt Cameron Pilley (AUS) 11-7, 12-10, 7-11, 11-8 (63m)
 Amr Shabana (EGY) v Thierry Lincou (FRA)
 Hisham Mohd Ashour (EGY) v Stephen Coppinger (RSA)
 Daryl Selby (ENG) v Borja Golan (ESP)
 Laurens Jan Anjema (NED) v Julian Illingworth (USA)
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