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Xavi, the heartbeat of Barca's game.
Barca play a game which fundamentally focuses on keeping the ball to hone technical ability, but do not underestimate the amount of hard work the players put in to fulfill their roles.
In possession, off-ball movement is key to creating space and opening passing lanes.
Off the ball, organised and intense pressing wins back possession and the impetus of play.
One of the most frustrating sights in recent weeks has been the number of times players have been spotted simply walking about the pitch.
Clearly the players are both mentally and physically fatigued, many coming off a busy summer at Euro 2012.
Much of the problem is owed to an approach to fitness training known as "periodisation."
Barcelona use a distinctive conditioning program which aims for the players to reach peak fitness towards the end of the season. The trade-off, however, is that form and fitness dips in Europe's winter months.
Indeed, Goal.com's Paul McDonald has pointed out that throughout the Guardiola years, February has proved troublesome.
Perhaps finally, Barca will pay more severe consequences for this fitness tactic.
Xavi, Barca's maestro, has particularly struggled for fitness.
The Catalan midfielder has been carrying a hamstring strain and bears the signs of an overworked player.
Without a fit Xavi, Barcelona's game has severely lacked tempo and fluidity, two hallmarks of successful "tiki-taka" football.
While Iniesta and Messi have continually provided the spark this season, one must not underestimate the influence of Barca's No. 6.