Biarritz coach John Isaacs admits Shane Williams' form has the French giants worried.
Isaac's men face wing wizard Williams in Saturday's Heineken Cup crunch quarter-final in San Sebastian.
The Ospreys predator scored tries in three of Wales' Six Nations clashes against Scotland, France and Italy. And Isaacs says the 2006 European runners-up are running scared of former World Player of the Year Williams ahead of the monster tie.
Isaacs revealed: "Shane is capable of making the difference with his actions. He comes alive on the pitch in open play and is a dangerous, big game player.
"We have to be careful about giving him loose ball and make sure our kick chase is good. We will have to keep the ball away from him as much as we can."
Williams will come face-to-face with American speedster Takudzwa Ngwenya, who famously destroyed Bryan Habana at the 2007 World Cup.
But Isaacs believes 33-year-old Williams, fit again after a shoulder complaint, will test the Zimbabwean-born star to the limit. "It will be a good battle but Shane is on a different level to Takudzwa," he said.
"Shane has good rugby knowledge and although Takudzwa is a formidable finisher, he is less intuitive than Shane as he is reasonably new to the game. But he can be a match-winner.
"He got a hat-trick against Gloucester and has speed on the outside. We hope he will be up for the game.
"But we can't just worry about Shane. The Ospreys are very strong and James Hook and Mike Phillips are also dangerous players.
"Our defence will have to be strong. We will have to do the basics well, make our tackles and ensure we don't turnover the ball."
Finals Edition: Listen to the best guide to the European Rugby Cup Finals weekend in Dublin, which sees Leinster Rugby take on Stade Francais Paris at the RDS on Friday night in the Amlin Challenge Cup Final and then ASM Clermont Auvergne tackle Toulon in an all-French affair at the Aviva Stadium on Saturday.