LONDON (AP) — Humbled by Celtic in the latest shock result in the Champions League, Barcelona travels to Russia this week as the competition favorites attempt for the second time to seal a place in the knockout stage.
The Champions League has thrown up a string of unpredictable scorelines this season and none bigger than Barca's 2-1 loss in Glasgow two weeks ago, preventing the Spanish giants joining FC Porto, Malaga and Manchester United in the last 16 with two games to spare.
Lionel Messi and his team still need one more victory to advance in Group G but achieving that at Spartak Moscow on Tuesday is far from guaranteed, given that Barcelona has only won once in six trips to Russia.
"We must not drop our guard or relax," Barcelona playmaker Andres Iniesta. "Up to this point, our start to the season has been sustained and sensational. But we must continue as we've begun."
While Barcelona is still a big favorite to progress, the same can't be said of a number of Europe's big guns.
Manchester City has been on the cusp of elimination for a month now and the fate of the English champions, who are bottom of Group D on just two points from four games, will be sealed if they fail to beat Real Madrid at home Wednesday in the most high-profile clash of the fifth round of group games.
A win for Madrid would likely take it through, although Jose Mourinho's side is also stumbling after picking up just one point from its double-header with silky German side Borussia Dortmund.
"We will play with practically our season on the line," Madrid defender Sergio Ramos said. "As a football player, those are the kind of games you live for."
Dortmund is the group leader and needs only a point at Ajax to advance.
Chelsea is engaged in a tough three-way fight for the two qualification spots in Group E, and a loss in Juventus on Tuesday will leave the holders' destiny out of their hands. Only one point separates Chelsea, Juventus and group leader Shakhtar Donetsk, which could qualify with a win at FC Nordsjaelland.
AC Milan needs a win in Anderlecht on Wednesday if the struggling Italian side isn't to face the serious prospect of elimination from Group C, while the winner of Tuesday's match between Valencia and last season's runner-up Bayern Munich will qualify in Group F.
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