It was yet another dramatic night in the Champions League as Barcelona lost its first game of the season while Chelsea was beaten at FC Basel. The tightest group of all, Group F, promises to bring mathematical headaches to all fans, while FC Porto squandered a great opportunity to take control of its own fate.
Here's what caught our eye in a fascinating round of action:
Group of Death goes down to the wire
Has anyone got a spare calculator? After Arsenal beat Marseille 2-0 and Borussia Dortmund beat Napoli 3-1, Group F is building up to be the deadliest Group of Death in history. With one game to go, Arsenal is on 12 points and Napoli and Dortmund have nine points. Realistically, all could still end up on 12 points after Matchday Six, with progression decided on results of a mini-league between the three.
If it beats pointless Marseille, Dortmund will qualify, given that it matches Arsenal on head-to-head and has the better of Napoli (1-2 in Italy, 3-1 in Germany). Arsenal beat Napoli 2-0 at the Emirates, which means the Italians need to score three to qualify. Arsenal can top Group F even if it loses by one goal in Italy.
Both Arsenal and Dortmund got off to perfect starts; Jack Wilshere curled in a goal inside one minute for Arsenal, while Marco Reus scored a fifth-minute penalty after Federico Fernandez manhandled Robert Lewandowski at a corner. That's where the first-half similarities ended.
Arsenal was coasting against an under-strength Marseille side, while Dortmund, though it had chances against Napoli - Lewandowski squandered two in the first half - was jittery. Mesut Ozil even missed a penalty for Arsenal, saved after trying a dummy mid-run-up.
This was no easy task for last year¹s runner-up Dortmund, not just because it had lost its previous three matches but it was also missing, through injury, all four defenders who started last season's Champions League final.
In their place were three midfielders and Sokratis, a summer signing whose error against Arsenal cost Dortmund on Matchday Four. The nervousness showed, at both ends of the pitch: Lewandowski was not the same player who destroyed Real Madrid on this pitch last April, while Napoli looked to play behind the makeshift fullbacks and at one stage found its way through only for Jose Callejon to strike the post.
Napoli needed a draw to qualify and continued to push for the equalizer - that left more gaps for Dortmund on the break, and early in the second half Reus and Lewandowski squandered more chances. Reus also hit a dipping free-kick that Reina tipped over, and the Spaniard kept out Mkhitaryan.
Dortmund thought its moment had come on the hour: first Gonzalo Higuain was 1-on-1 against Roman Weidenfeller, only for the goalkeeper to make a brilliant save. From the resulting counter, Reus released Jakob Blaszczykowski to double Dortmund's lead.
There was still danger, as one Napoli goal would level up Dortmund¹s goal difference advantage. And that came after substitute Lorenzo Insigne chipped over Weidenfeller after an error from Sebastien Kehl. In a hugely open game, the chances kept coming at both ends: just minutes after Reina brilliantly stopped Pierre-Emerick Aubameyang, he could do nothing to stop the same player chipping home from Lewandowski¹s through-ball. It was 3-1 Dortmund, but one more goal from Napoli would still have given it the edge on away goals, so tight is the group.
That did not come - and the final whistle was met with as much relief as delight for the German side. Meanwhile, Wilshere scored a second as Arsenal eased to a relatively comfortable home victory. And yet, all this maths may yet prove irrelevant.
It's worth noting that on current form, Napoli would struggle to beat Arsenal, even at home.
Barcelona boss Martino facing more pressure
For all the talk around Barcelona coach Tata Martino, it still comes as a surprise to learn that the 2-1 defeat at Ajax was his first loss of the season, in game No. 21. OK, so Tata does not equal the unbeaten record set by Pep Guardiola's team of 2011-12, but of more concern will be the listless performance by a Barcelona side that was missing key men at both ends: Lionel Messi and Victor Valdes.
While the game at Ajax was a chance, as vice president Josep Maria Bartomeu put it, "to show that we can cope without Messi," the loss of Valdes seemed more telling.
Ajax -- who, it must be said, deserved the win (and was without key players itself, including skipper Siem de Jong) -- took advantage of a weak parry by Jose Pinto for Danny Hoesen to make it 2-0. That proved to be the key goal, as early in the second half, Neymar won a penalty, which was converted by Xavi, and Ajax was reduced to 10 men after the dismissal of Joel Veltman.
Coach Frank de Boer made some sharp changes, taking off Lasse Schone and shoring things up with Stefano Denswil. It worked: the 10 men held off Barcelona's challenge for 40 minutes, although the result might not yet be enough to see Ajax into the next round: AC Milan beat Celtic 3-0 and is one point ahead of the Dutch champion.
The two sides face off on Matchday Six, though Milan, who hosts the game, holds the advantage. Celtic is out of Europe after that loss.
Porto fails to capitalize on Atletico favor
Atletico Madrid changed half of its outfield starting players and ended its 100 percent record, though remains unbeaten in Group G after drawing 1-1 at Zenit St Petersburg. That result allowed FC Porto to have control of its own fate but it slipped up, drawing 1-1 at home to previously pointless (and goalless) competition debutant Austria Vienna.
Now Porto needs to improve on Zenit's result against Vienna on Matchday Six, which is no easy task given the game is at Atletico. A chance missed for Porto, who has not yet a single home match in this group.
Atletico, meanwhile, showed in the early kickoff that even without the rested Diego Costa, David Villa, Arda Turan, Tiago and Diego Godin, it can look very dangerous. This was an opportunity for some of Atletico's second string to make a claim, among them Josuha Guilavogui, the France midfielder who had played for only 38 minutes since signing for €10 million from Saint-Etienne on transfer deadline day.
"It's one thing to play well for one team, it's another to do the same for us," coach Diego Simeone said ominously of Guilavogui's lack of first-team action.
Playing as the deep-lying holding midfielder, Guilavogui took his chance to impress and his strength and quick passing might see him get more playing time in the second half of the season. Not sure if the same will be said of Toby Alderweireld, another player barely used since joining in the summer. His own goal leveled up for Zenit and added to the question marks over goalkeeper Thibaut Courtois's current form. His slight dip is not anything like that of Manchester City's Joe Hart, for example -- and in part it's because his standard of the last two years has been so ridiculously high -- but given the form of his back-up in the Belgium team, Simon Mignolet, the timing is not ideal.
Still, this was a positive night for Simeone, and for Guilavogui too.
Off-color Chelsea still qualifies
Maybe it was not how Jose Mourinho wanted it, but at this stage of the competition, it's not about playing well - lucky for him, because Chelsea was poor at FC Basel, and the Swiss champion¹s late winner from Mo Salah completed an impressive double over the Blues.
"It was a bad performance and a deserved defeat," moaned Mourinho after the game.
At least his team is through: Schalke¹s 0-0 draw with Steaua Bucharest leaves Chelsea one point ahead of Basel and two of Schalke, who face each other in Gelsenkirchen on Matchday Six. The German team will need to win at home to qualify, while Chelsea will need to beat Steaua to finish top. To add to Mourinho's headache, Samuel Eto'o was stretchered off with a groin injury.