Munich, July 3 (IANS): Bayern Munich head coach Pep Guardiola has returned to work with his team and predicted a difficult season up ahead, in which the German football giants aspire to win the Bundesliga for the fourth consecutive time.

"No team has ever won the Bundesliga four times in a row," Efe quoted Guardiola as saying on Thursday during the Spaniard's first post-summer vacation press conference.

Several teams represent a challenge against Bayern for next season, the Spanish coach said, listing teams such as Borussia Dortmund, Borussia Monchengladbach, Bayer Leverkusen and FC Schalke 04.

"Dortmund has a new coach and that could give them a boost. Gladbach are now coming to show their capabilities. Leverkusen has a second season with (coach Roger) Schmidt who now knows his players better. Wolfsburg had a great season and Schalke need to come back," Guardiola added.

The former FC Barcelona manager previously said that although winning titles is important, the key to any season is constant improvement every week, explaining that his dream is to reach the quarter-finals or the semi-finals of the European Champions League with a full squad.

Bayern already reached those Champions League stages last season, however several of their players suffered injuries, like wingers Franck Ribery and Arjen Robben.

Apparently, the fate of those players was a driving reason behind signing Brazilian player Douglas Costa, who can play on both wings.

Guardiola downplayed that Costa was playing in the weaker Ukrainian Premier League before joining Bayern from Shakhtar Donetsk.

"That doesn't matter. (Sergio) Busquets and Pedro (Rodríguez) played in the fourth division and then they won the Champions League and were world champions at Barcelona! ... The important thing is not the league you come from, but your quality," Guardiola explained.