The Union battled out a 0-0 draw against perennial Peruvian champions Alianza in Lima on Tuesday while the Fire earned a 1-1 draw with New York City FC in MLS last weekend.
Match preview
The importance of Philadelphia's upcoming second leg at home to Alianza next week cannot be understated - even if the professionalism of the entire continental competition was undermined by a dog running onto the pitch and biting the ball during the first leg of the round of 16 tie in Peru.
Philadelphia made it all the way to the semi-finals on the only other occasion when the Pennsylvania outfit have participated in the Champions League, losing home and away to the Mexican giants Club America.
The Union, who were beaten on penalties by Los Angeles FC in an all-time classic MLS Cup final last year, returned to league action with a 4-1 thrashing of Columbus Crew a fortnight ago.
However, Jim Curtin's side lost 2-0 to Phil Neville's Inter Miami last weekend, conceding a pair of crisp long-range strikes from Corentin Jean and Robert Taylor on either side of half time.
Philadelphia have won their last 12 MLS matches at Subaru Park, scoring an outrageous 48 goals during this sequence, and they are unbeaten on home turf since December 2021.
The Fire finished a yawning 28 points behind Philadelphia last season, sitting 12th in the Eastern Conference and 24th overall - the sixth time in the last eight seasons that the club have resided in the bottom 20 of the combined divisions.
Ezra Hendrickson's side have only played one match of the new league season, with Fabian Herbers's second-half volley flying through a crowded box to cancel out Gabriel Pereira's opener for NYCFC last weekend.
Chicago made the biggest profit during the off-season, falling victim to the riches of the Premier League as they lost last season's top scorer Jhon Duran to Aston Villa and first-choice goalkeeper Gabriel Slonina to Chelsea.
Philadelphia also came out of the hiatus in the black but the departure of 19-year-old Paxten Aaronson - the brother of Leeds United's Brenden Aaronson - to Eintracht Frankfurt is nowhere near as significant in the short term as the losses suffered by the Fire.
Philadelphia Union Major League Soccer form:
Philadelphia Union form (all competitions):
Chicago Fire Major League Soccer form:
Team News
Curtin has named the exact same XI in both of the Union's MLS games this season but changed eight of the 10 outfield players for the trip to Peru.
A fully fit squad allows Curtin, the second-longest serving manager in the division, to continue picking one team for the Champions League and another in domestic competition.
Hendrickson has not been so lucky with injury after Federico Navarro and Jairo Torres hobbled off in the first half of the Fire's season opener with leg issues.
Spencer Richey's young understudy between the sticks Chris Brady also has complaints with his right upper leg, while centre-back Mauricio Pineda is a doubt with a similar issue.
Both Sergio Oregel (illness) and Justin Reynolds (ankle) are unlikely to feature in the daunting trip to Philadelphia.
Philadelphia Union possible starting lineup:
Blake; Mbaizo, Glesnes, Elliott, Wagner; Flach, Martinez, Bedoya; Gazdag; Carranza, Uhre
Chicago Fire possible starting lineup:
Richey; Souquet, Teran, Czichos, Navarro; Gimenez, Herbers; Gutierrez, Shaqiri, Mueller; Przybylko
We say: Philadelphia Union 3-1 Chicago Fire
Philadelphia may have an eye on Alianza's visit but one of the favourites to top the Eastern Conference are unlikely to have their winning home run ended by a gutted Chicago Fire side this weekend.
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