(Reuters) - World number eight Gael Monfils has been ruled out of France's Davis Cup semi-final against Croatia in Zadar because of a knee injury, the French tennis federation said on Wednesday.

The U.S. Open semi-finalist is replaced by Richard Gasquet, who will play in Friday's singles along with Lucas Pouille.

France were already without the injured Jo-Wilfried Tsonga.

Britain host Argentina in the other semi-final.

(Reporting by Julien Pretot; Editing by Ken Ferris)