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The fourth round of Champions League group stage fixtures is here, and this week's matches have the potential to prove decisive for a number of groups.

Halfway through the group stage -- the top two of each four-team group advance to the knockout phase, third is relegated to the Europa League and fourth is eliminated -- some of Europe's biggest teams look set to cruise on through while others are facing a fight to qualify.

Inter Milan and Real Madrid are arguably the two teams most at risk this Wednesday with the Serie A outfit bottom of Group B with two points from three matches and Zinedine Zidane's men visiting San Siro this week.

Elsewhere, Manchester City, Bayern Munich and Liverpool are pretty much through, it is just a question of whether they top their groups

This is what we are looking forward to this midweek.

GROUP B: Inter Milan must kill or be killed off

Inter's clash with Real at San Siro on Wednesday is arguably the tie of round along with Paris Saint-Germain against RB Leipzig on Tuesday. The pair played out a pulsating five-goal encounter last time out which the Spaniards ultimately came out on top in.

With Borussia Monchengladbach leading the way in Group B and at home against a Shakhtar Donetsk side they demolished 6-0 away from home last time, both sides will be expecting another German win.

Regardless of the result Inter need a win to keep themselves in contention for a berth in the latter stages if they want to avoid dropping into the Europa League.

Both are in mixed domestic form with just two wins from their last five apiece in Serie A and La Liga respectively. Inter have only lost once -- at home to bitter rivals AC Milan -- while Real have been beaten twice, by Cadiz at home and Valencia away.

A win would be big for both in terms of their latter stage ambitions but anything other than a win will make life extremely difficult for Inter.

GROUP C: Marseille close on unwanted record

With City away at Olympiacos and still perfect on the season with nine goals scored and just one conceded, Pep Guardiola's men should confirm their place in the knockout rounds on Wednesday in Piraeus.

The Greeks profited late on against a poor Marseille side but have been soundly beaten by both City and Porto since that opening day victory.

The real intrigue in Group C this week is at Marseille where Andre Villas-Boas' men will welcome former club Porto to Stade Velodrome hoping to end an ugly run of 12 consecutive Champions League defeats dating back to 2012.

If OM cannot stop the rot, they will set a new record for losses in the competition after equaling Anderlecht's unwanted figure from the 2003-05 period.

Marseille have improved lately in Ligue 1 with three wins and three draws to go sixth in the table with two games in hand and a six-point gap to leaders PSG, but they have not played since a 1-0 win away at Strasbourg before the international break due to COVID-19 outbreaks.

The 1992-93 winners must win against Porto to stand any chance of salvaging something from their return to the big time and to save French football from another humiliating set of continental results.

GROUP D: Ajax and Atalanta vying for second place

Liverpool against Atalanta in Group D at Anfield on Wednesday is a headline clash, but Jurgen Klopp's men are already five points ahead of the entertaining Italians.

Instead, it is Atalanta's final day trip to Amsterdam to face Ajax that should prove decisive in the battle for second place.

The Dutch outfit are at home to a Midtjylland side yet to even register a point so Gian Piero Gasperini's men will be wary of being pushed towards the Europa League berth if they do not get a result in Liverpool.

With the Danes visiting Gewiss Stadium next week, a draw would not be the worst result for the Serie A outfit.

GROUP A: Atletico Madrid lead chase for second

With defending champions Bayern at home to Red Bull Salzburg and marching on at the top of Group A, few are expecting the Austrians to pull off a shock.

Consequently, Atletico Madrid are playing for second spot with Bayern to come next week in what could be a chance to close the gap on the Germans if Salzburg manage to take a point or more at Allianz Arena.

Lokomotiv Moscow have saved their best performances for Europe this season and could claim third place and Europa League football ahead of Salzburg but that will likely be decided by next week's meeting between the two in Russia.

After beating Barcelona at home in La Liga over the weekend, Diego Simeone's men will feel confident that they can keep the pressure on Bayern.