Defending champions Real Madrid suffered a shock defeat at the hands of underdogs CSKA Moscow even as Jose Mourinho's Manchester United continued to misfire on Tuesday.
Holders Madrid slumped to a rare defeat as they went down 1-0 in Moscow, while United played out a dull 0-0 draw with the visiting Valencia at Old Trafford.
Another heavyweights in action on Tuesday, Bayern Munich were also held by Ajax. But it was a happier evening for Pep Guardiola's Manchester City as they beat Hoffenheim 2-1. Fellow title contenders Juventus were also comfortable winners thanks to Paulo Dybala's hat-trick.
Real were not expected to struggle in Group G as they look to continue their recent dominance of the Champions League, but Julen Lopetegui's side went behind inside two minutes in Moscow following a mistake by Toni Kroos.
The German's drilled backpass turned into a gift for Nikola Vlasic, and the winger on loan from Everton fired past Keylor Navas to put CSKA ahead.
Casemiro hit the post in the first half -- one of three attempts that found the woodwork for the visitors -- but Real were weakened by the absences of Gareth Bale and Sergio Ramos, while Luka Modric started on the bench.
CSKA held on, despite having goalkeeper Igor Akinfeev sent off in injury time, and they move above Real to the top of Group G.
"I am sure we will soon be scoring again and winning again," said Lopetegui, whose team have not found the net in three matches.
It was only Madrid's third defeat in the Champions League group stage this decade, but Real will nevertheless expect to bounce back next against Czech champions Viktoria Plzen, who were crushed 5-0 by Roma.
Edin Dzeko scored a hat-trick for the Italian side, with Cengiz Under and Justin Kluivert also netting.
At Old Trafford, the pressure continued to build on Mourinho as United produced another insipid attacking display in their goalless stalemate with Valencia.
They are now four games without a win, and Mourinho's decision to recall Alexis Sanchez -- after dropping the Chilean for the weekend defeat at West Ham United -- failed to pay off.
"Our attacking players aren't in their best moments of confidence and individual level," moaned Mourinho.
"We thought with three or four chances we would score and win the game."
Mourinho's side are second in Group H, two points behind Juventus.
Dybala shines in Ronaldo absence
Juve eased to victory against 10-man Young Boys in Turin, with Dybala scoring a hat-trick to take centre stage in the absence of Cristiano Ronaldo, who was serving a one-game ban.
"We were without Cristiano, and at times it is easier with him in the side, but we have to be a great team both with and without him," said Dybala.
Guardiola saw Manchester City come from behind to beat Hoffenheim in Germany in Group F.
David Silva scored three minutes from time to get the Premier League champions back on track after they started their campaign with a 2-1 home defeat against Lyon.
Ishak Belfodil had put Hoffenheim ahead after just 44 seconds, but Sergio Aguero equalised in the eighth minute before Silva's late winner.
"This competition isn't always a red carpet. Manchester City is a fantastic club, but we don't have a history of having to fight and we fought for absolutely everything here," said Guardiola.
"This is one of the happiest days I have had at this club as it will help us going forward."
City next face Shakhtar Donetsk, who squandered a two-goal lead to draw 2-2 with Lyon in France, in a game played behind closed doors.
Brazilian forward Junior Moraes scored twice for Shakhtar, but Lyon fought back to draw with goals from Moussa Dembele and Leo Dubois.
It was a good result for Lyon, whose fans were missing as a punishment for trouble at a game against CSKA last season, and they now top the group.
Meanwhile, Bayern could not hang on to the early lead given to them against Ajax when Mats Hummels headed in Arjen Robben's cross. Young full-back Noussair Mazraoui equalised to give the Dutch side a 1-1 draw.
Those sides are level atop Group E ahead of Benfica, who won 3-2 in a thriller at AEK Athens with Alfa Semedo scoring the deciding goal.
(With AFP inputs)