Bayern Munich begins the quest for its sixth European Cup at Olympiakos on Wednesday, while Arsenal opens its 18th successive Champions League campaign at Dinamo Zagreb.
Dinamo, back in the competition after a two-year absence, is on a 41-game unbeaten streak across all competitions. The Croatian champion has never previously qualified from the group stage in four previous attempts.
Olympiakos claimed big upsets against Atletico Madrid and Juventus at its intimidating Georgios Karaiskakis Stadium last season, though the wins weren't enough to ensure progress beyond the group stage.
Bayern, the 2013 champion, is also hoping to go at least one stage further after semifinal exits the past two seasons.
Some things to know about Wednesday's Group F matches:
Pep Guardiola's tenure as Bayern coach will be judged on success in Europe this season, likely his last with the Bavarian club.
After taking over Jupp Heynckes' treble winning side in 2013, Guardiola has enjoyed overwhelming Bundesliga success while crashing out in successive seasons to eventual champions in the Champions League semifinals. Last May's 5-3 aggregate loss to Barcelona was at least an improvement on the 5-0 defeat to Real Madrid the previous year.
Now in the final year of his contract, with no indication yet of whether he will extend, Guardiola is under pressure to lead Bayern to its sixth European title in his third attempt.
Arturo Vidal, Douglas Costa and Kingsley Coman were signed to help him achieve that aim, following the arrivals of Robert Lewandowski, Xabi Alonso and Juan Bernat the previous year, and Mario Goetze and Thiago Alcantara the year before that.
STILL IN SECOND GEAR
Despite four wins from four in the Bundesliga, Bayern is not yet fully convincing in the league. Only a controversial penalty decision allowed Thomas Mueller score Bayern's last-minute winner against Augsburg on Saturday, while the side needed another 90th-minute strike from Robert Lewandowski to beat Hoffenheim by the same 2-1 score.
Guardiola bemoaned his side's tendency to coast through periods of games and he warned his players that they had to play for 90 minutes to appease the fans.
Bayern will have to crank up a gear in Piraeus.
Wingers Arjen Robben and Franck Ribery are ruled out through injury, while attacking midfielder Mario Goetze should have overcome a muscle strain that kept him out of action Saturday.
Olympiakos will again be counting on home support and Spanish goalkeeper Roberto.
Coach Marco Silva has kept Roberto and the core of his squad, while adding former Argentina international Esteban Cambiasso.
Midfielder Pajtim Kasami could start, filling a gap left by the suspended Luka Milivojevic. Portuguese striker Hernani, who came to Greece on a summer loan from Porto, is also likely to make the selection, while Argentine midfielder Alejandro Dominguez will again be handed a key role in midfield after some standout performances.
Dominguez scored Saturday as Olympiakos cruised to its third straight win, 3-1 over Platanias, yet Silva was far from happy.
''We lost concentration after the goals because the players had Wednesday's game on their mind. I didn't like it but I suppose it was unavoidable,'' Silva said.
ARSENAL'S STRIKING ISSUE
Arsenal manager Arsene Wenger came under intense scrutiny from the English media for not signing a striker in the summer transfer window, with doubts about Theo Walcott and Olivier Giroud's ability to consistently deliver at the highest level and Danny Welbeck out until Christmas with a knee injury.
But Walcott and Giroud vindicated Wenger on Saturday, each finding the back of the net against Stoke to lead Arsenal to a 2-0 win, its first at home this season.
It remains to be seen which player he starts in attack. Walcott led the line against Stoke, with Giroud replacing him 15 minutes from the end.
DINAMO'S RUN IN JEOPARDY
Dinamo hasn't lost a game since falling 1-0 to Astra Giurgiu in the Europa League group stage in November 2014, but the side was beaten in both legs of the Champions League third qualifying round in 2006. They were the only previous meetings between the sides.
The Croatian club has never emerged from the group stage in either the Europa League or Champions League and is 15 games without a win in the latter, not including qualifiers.
Dinamo has yet to win against Premier League opponents, drawing twice and losing seven times.
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