Real Madrid may head into El Clasico 11 points off LaLiga's summit but leaders Barcelona should be wary of a wounded animal, says Camp Nou icon Xavi.

Fresh from securing Club World Cup glory in Abu Dhabi, Madrid host the first league Clasico of 2017-18 on Saturday, desperate to close the gap on their fierce rivals.

Reigning Spanish and European champions Madrid have enjoyed a stellar 2017, which Xavi suggests could have led to a dip in performances in the early months of the current campaign, but he warned his old club that nothing can be taken for granted in El Clasico.

"I don't know if they have relaxed or not but they won LaLiga, the Champions League and the Supercopa [de Espana] so assuredly," he told Catalunya Radio.

"That could be the reason for them not being at their level, but Real Madrid are difficult to kill off."

The Champions League last-16 draw threw Madrid together with Paris Saint-Germain in the tie of the round, and Xavi believes whoever prevails from that mouth-watering clash will be favourites to go on and lift the trophy.

"In the Champions League, [Manchester] City, Barca, Real Madrid and Bayern [Munich] under Heynckes now," he said of potential challengers for the trophy.

"The one who eliminates the other in the PSG-Madrid game will be a favourite because that is a massive exit. Barca will have nothing easy."