By Iain Rogers
MADRID, Nov 28 (Reuters) - Athletic Bilbao host Barcelona at their new San Mames stadium on Sunday sniffing an upset after the La Liga leaders suffered their first defeat of the season at Ajax Amsterdam in the Champions League on Tuesday.
Unbeaten in domestic competition and three points clear of second-placed Atletico Madrid at the top after 14 matches, Barca have lost several key players, including Argentina forward Lionel Messi and Spain goalkeeper Victor Valdes, to injury.
Bilbao, who are fifth, will hope to emulate Ajax's attacking verve and fighting spirit after the Dutch champions took the game to their illustrious visitors in the first half and then held on for a memorable win despite losing Joel Veltman to a red card shortly after the break.
Bilbao have been in fine form at their new stadium this term, winning five and drawing two, while Barca's last two visits to the Basque club's old San Mames arena, now demolished, both ended in 2-2 stalemates.
Midfielder Ander Herrera, who scored in each of those games, said the key to Sunday's clash (2000 GMT) will be to avoid the ultra-defensive tactics most of Barca's opponents rely on.
"It is difficult to imagine Barca slipping up in two games in a row but that is our task," Herrera was quoted as saying in Marca sports daily on Thursday.
"To beat them we have to play with complete freedom, without hanging back in defence," added the 24-year-old.
"We have the capacity to play much better and with confidence and the right results excellence will follow."
Barca could find themselves level on points with Atletico and only three ahead of third-placed arch-rivals Real Madrid going into the Bilbao game if the city neighbours win their matches on Saturday.
Atletico play at promoted Elche (1500) and Real are at home to struggling Real Valladolid (1900).
The Barca players were self-critical after the Ajax defeat and bemoaned a lack of intensity in the early stages that allowed their opponents to seize the initiative.
"The team lifted their game in the second half but we are left with a bad feeling about the first," playmaker Cesc Fabregas told reporters.
"We knew we were not on good form on the pitch and we had the sense we were running a lot, more than in other matches, but running badly," added the former Arsenal captain.
"And that is worse. Tactically they were better and we did not know where to pick them up."
Villarreal, a point ahead of Bilbao in fourth, play at home to Malaga on Friday (1930).
Both Malaga coach Bernd Schuster and his Valencia counterpart Miroslav Djukic, whose side host Osasuna on Sunday (1800), are under pressure after poor starts to the season. (Editing by Amlan Chakraborty)