Como enjoyed a dream Champions League debut Thursday as the Italian club trounced RB Leipzig 4-1 at home, but fellow new boys Sabah were crushed 4-0 at Manchester United.
Six-time winners Bayern Munich opened their group phase campaign with a 5-0 thrashing of Norway's Bodo/Glimt while Fenerbahçe made their long-awaited return to the competition with a 1-1 draw at home with AS Roma.
Como's first-ever qualification to Europe's elite club competition meant a race against time to get their lakeside Stadio Sinigaglia home upgraded to meet UEFA guidelines.
Failure to do so would have meant playing their home games 200km away at the home of Sassuolo, but Cesc Fabregas's side were spared that and celebrated in style.
Marti Baturina got the party started with a 15th-minute opener before Taso Douvikas doubled their lead.
Assane Diao made it 3-0 early in the second half before Andrija Maksimovic pulled one back for the visitors, but Maximo Perrone wrapped it up in stoppage time.
Azerbaijan champions Sabah did not even exist a decade ago, but after winning the domestic title they moved into the qualifying rounds and beat Israel's Hapoel Beer-Sheva to proudly take their place in the group stage alongside Europe's big guns.
A visit to Old Trafford for United's return to the Champions League after a two-season absence proved a step too far, though, as they were rolled over by Michael Carrick's side.
Matheus Cunha began the rout in the 27th minute before Bruno Fernandes and Benjamin Sesko struck shortly before halftime. Lisandro Martinez added United's fourth.
Bayern were frustrated before halftime by Bodo/Glimt but cruised once Jamal Musiala smashed in the opener two minutes after halftime and the floodgates opened.
Harry Kane headed the second on the hour – his 34th goal in 39 Champions League appearances for Bayern – and Alphonso Davies made it 3-0 with a powerful half-volley.
With Vincent Kompany's side in full flow, Michael Olise completed the scoring with a superb late brace.
Roma took the lead in Istanbul in the 39th minute when Donyell Malen rolled a pass to Bryan Cristante, who beat Ederson with a bouncing finish. It was Cristante's first goal in the Champions League proper, 15 years since making his debut in the competition as a 16-year-old for AC Milan.
Fenerbahçe, back in the Champions League after an 18-year absence, rallied through and equalized three minutes after halftime when Mason Greenwood slipped a pass through to Archie Brown, who clipped a finish over Mile Svilar.
The hosts almost won it late on when Nathan Ake headed against the Roma crossbar.
PSV Eindhoven avoided a sixth successive matchday one defeat in the competition as they drew 1-1 at home to Shakhtar Donetsk.
The Dutch club trailed at halftime after Gleiker Mendoza cut in from wide and sent a stunning low shot beyond the dive of Matej Kovar. However, Sergino Dest's deft finish soon after the break ensured they began with a point.
Ten-man Slavia Prague suffered heartbreak at home to Lens as they conceded twice in stoppage time to lose 3-2.
Mikulas Konecny was sent off for the hosts just after halftime after a tackle was reviewed by VAR, but minutes later Danijel Sturm put the hosts ahead.
Abdallah Sima broke Slavia's resistance with an equalizer, only for Sturm to send the home crowd into raptures with his second in the 88th minute.
Florian Thauvin struck in the 91st minute and there was still time for Lens to snatch victory with a pinpoint strike by Ruben Aguilar.