Arsenal take a 1-0 lead to Italy on Wednesday for the second leg of their Champions League play-off against Udinese, while Bayern Munich and Lyon also look to clinch places in the group stage.
Arsene Wenger's men overcame the departure of Cesc Fabregas to Barcelona, and the absence of numerous key players, to take an early lead in the first-leg thanks to Theo Walcott.
However, Udinese were unlucky not to nab an away goal and will be hopeful of turning the tie around at home, especially as the English outfit's fragile confidence took another hit at the weekend when they went down 2-0 at home to Liverpool.
That result meant the Gunners have now failed to score in their first two league games of a season for the first time in 42 years, while their hopes of stopping Udinese from scoring have been hit by a back injury to Laurent Koscielny.
After the Liverpool reverse, a downbeat Wenger decribed the Udinese return as being "of big importance," but it is diffuclt to know where the Gunners would go next if they failed to qualify for the group stages.
Robin van Persie and Samir Nasri are available again after suspension, and Nasri could play unless his mooted transfer to Manchester City is completed before Wednesday.
Udinese do not begin their domestic campaign until next weekend, so Francesco Guidolin's side have been able to focus all their energy on beating Arsenal and returning to the Champions League group stages for the first time since 2005-06.
Meanwhile, Bayern Munich -- who dream of going all the way to a final which will be held in their own Allianz Arena -- have more than one foot in the group stages after beating FC Zurich 2-0 in last week's first leg.
The Bavarians head to Switzerland further boosted by a superb performance in beating Hamburg 5-0 in the Bundesliga at the weekend.
Lyon prepared for the second leg of their tie with Rubin Kazan with an underwhelming 1-1 draw at Brest, but Remi Garde's side impressed in their 3-1 win over the Russians last Tuesday.
Rubin, who lie 10 points behind Russian league leaders CSKA Moscow with just nine games to go, have considerable selection problems just now, although new Paraguayan signing Nelson Valdez could feature.
In Spain, 2006 semi-finalists Villarreal must overturn a 1-0 first-leg deficit against unfancied Odense.
Juan Carlos Garrido's side, who had no game at the weekend after a strike put paid to the opening round of games in La Liga, should still have enough quality to advance.
"We are all aware of the situation because our season depends in large part on winning this tie," said Villarreal's Argentine striker Marco Ruben ahead of the clash at El Madrigal.
"In fact, it's probably a good thing that we didn't play at the weekend because we have been able to focus all our attention on the Champions League, which is fundamental to our season."
Two-time former European Cup winners Benfica are well-placed to overcome the challenge of Twente Enschede after drawing 2-2 in the first leg in the Netherlands.
Belgian champions Genk must turn around a 2-1 first-leg deficit against the Israelis of Maccabi Haifa without coach Frank Vercauteren, who has left to take charge of Al-Jazira in the United Arab Emirates.
Viktoria Plzen and Dinamo Zagreb should go through after big first-leg wins, but APOEL versus Wisla Krakow and Sturm Graz's tie against BATE Borisov are finely poised.