The three teams with 100 percent records in this season's Champions League can secure their places in the knockout stage this week with two games to spare.
German heavyweights Bayern Munich and Borussia Dortmund -- who contested the 2013 final -- plus holders Real Madrid have all been in imperious form in the competition and will be confident of making it four wins from four games as they take on opponents they beat away from home a fortnight ago.
Real Madrid, now top of La Liga, are on a club record of 11 straight wins in all competitions as they host five-times European champions Liverpool in Group B, having beaten them 3-0 at Anfield on matchday three.
The main interest in that group appears to be who finishes second, with three teams all on three points. Basle and Ludogorets play their return game in Switzerland on Tuesday after the Bulgarians won 1-0 at home.
"We're still very much in the group to qualify, which is the objective," Liverpool manager Brendan Rodgers said ahead of a daunting trip to the Spanish capital.
"It's a big ask playing against arguably the best team in the world at the moment. But we'll go into it with confidence."
Bayern host AS Roma in Group E on Wednesday after an astonishing 7-1 away victory against them in Italy, and will be confident of taking another three points.
Roma's defeat offered new hope to winless Manchester City, who let a 2-0 lead slip away to CSKA Moscow and now need to win the return in England and hope the Italians lose again.
Dortmund are having a strange season in which they struggle domestically while brushing aside all opposition in Group D.
Beaten by Bayern in the Bundesliga on Saturday, they have dropped into the relegation places, but play at home to Galatasaray on Tuesday looking for a repeat of their 4-0 success in Turkey.
If Juergen Klopp's team and Arsenal, who are at home to Anderlecht, both win, they will each have qualified with two games to spare.
The four German representatives have lost only one game out of 12 between them so far, which was Bayer Leverkusen's 1-0 defeat to Monaco on match-day one.
They still top Group C, but a second loss, away to Zenit St Petersburg on Tuesday, would put the Russians ahead of them. Second-placed Monaco, away to Benfica, have yet to concede a goal and are also very much in contention.
The fourth German and English teams, Chelsea and Schalke, hold the top two spots in Group G ahead of Wednesday's matches.
Chelsea visit Slovenia to play Maribor, who they thrashed 6-0 in London, while second-placed Schalke know Sporting Lisbon have a poor record against German opposition, despite only having beaten them 4-3 in Gelsenkirchen with the help of a last-minute penalty.
Barcelona will want to bounce back from Saturday's shock defeat at home to Celta Vigo when they visit Ajax in Group F, in which they trail Paris St Germain by a point.
Four points ahead of Ajax, they would be well-placed to qualify with another win, as would PSG if they win as expected at home to APOEL.
Manager Laurent Blanc has said they must do that without talismanic striker Zlatan Ibrahimovic, who suffered a heel injury in September and has not played since.
He is due to return in the middle of this month.
Group A remains the closest, so much so that two home wins on Tuesday would bring all four teams level on six points.
Juventus and Malmo are those home teams, facing Olympiakos and last season's finalists Atletico Madrid respectively.
In Wednesday's other matches Group H leaders Porto travel to Athletic Bilbao and Shakhtar Donetsk host BATE Borisov, who they humiliated 7-0 in Belarus when Luiz Adriano scored five times.
