Atletico Madrid defender Antonio Lopez believes his side's Champions League third qualifying-round clash with Schalke is too close to call.
Atletico, making their first appearance in the competition for more than a decade, have been handed a tough first hurdle in the form of the Bundesliga side who reached the quarter-finals last season.
The Spaniards travel to Gelsenkirchen for the first leg on Wednesday and Lopez knows his team will have their work cut out to make the group stages.
'I think that the tie is 50-50,' the Spain international said. 'There will be a great atmosphere, but we hope that the game will turn out well for us.
'We are aware that we are going to face a difficult team but we must focus on ourselves. We have to plan, read the game well and impose our play on the match.'
Atletico will be without key striker Sergio Aguero, who is with Argentina at the Olympics.
Schalke have a new-look line-up from the side which was knocked out by Barcelona last time round.
Big-name summer signings Jefferson Farfan and Orlando Engelaar have added firepower up front and energy in midfield, while there is a new name at the helm too in Fred Rutten.
Captain Marcelo Bordon believes the result against Atletico will shape the Royal Blues' entire campaign.
'We have a cup final straight away which can decide the whole season and we have got to beat Atletico,' he said.
Rutten will be without regular goalkeeper Manuel Neuer, while Rafinha is at the Beijing Games with Brazil.
New Barcelona coach Pep Guardiola, who is aiming to end a two-year silverware drought for the Catalans, cuts his European teeth at home to Polish outfit Wisla Krakow.
Reports in Spain suggest he is set to include Samuel Eto'o in his squad, despite the striker being told he could leave the club earlier this summer.
Should Eto'o take part in Wednesday's game it would mean the Cameroon international would be ineligible to represent another side in this season's competition and could therefore affect any possible sale.
Summer signing Dani Alves knows how important it will be for Barcelona to put on a show for the home fans after the disappointment of the past two seasons.
He said: 'The home factor has to be fundamental. We must please our fans and win. I am anxious to see the reception. Our opponents must feel how difficult it is to play here. We are all together and must pull in the same direction.'
New FC Twente coach Steve McClaren will go head to head with some familiar faces when his side welcome Arsenal to Enschede.
The Eredivise outfit surprisingly pipped Ajax to the final Champions League qualifying spot last season, but have since seen former boss Rutten and influential midfielder Engelaar depart to Schalke.
Former England manager McClaren said: 'In the two matches against Arsenal we are not just an underdog, we are a very big underdog. Nobody will expect FC Twente to beat Arsenal.
'On the other side all the ingredients are there for a big shock. We have nothing to lose.'
Marseille president Pape Diouf, meanwhile, has warned the French side not to underestimate their Norwegian opponents Brann Bergen.
Diouf said: 'Logically, we should be able to knock them out. But they are in a championship in which they have already played 17 games. We have to deal with that.
'They certainly aren't a European heavyweight but we must go into these games with application, desire and a strong will.'
Baky Kone and Vitorino Hilton should both be fit for OM despite picking up knocks in the 4-4 draw with Rennes on Saturday.
Among the other ties, last season's surprise quarter-finalists Fenerbahce travel to Partizan Belgrade, while Turkish rivals Galatasaray also face eastern European opposition in the form of Steaua Bucharest.
Elsewhere, 2005 Champions League winners Liverpool travel to Standard Liege of Belgium and Juventus return to Champions League competition against Slovakian side Petrzalka, while Greek duo Olympiacos and Panathinaikos face trips to Anorthosis and Sparta Prague respectively.