Nadal out of Spain's Davis Cup semi-final

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Nadal retired while trailing his US Open last-four clash with Juan Martin del Potro by two sets to love, citing pain in his right knee.

The 17-time grand slam champion confirmed he would miss Spain's trip to Lille for its Davis Cup tie beginning on 15 September (AEST).

"Hello everyone, on the way home, in addition to having missed the opportunity to play a grand slam final here in New York, you will know that I finally cannot play Davis Cup this weekend in Lille against France," Nadal wrote on Twitter.

"I had made a good schedule thinking about my goals, including the Davis Cup, one of the reasons for not playing Cincinnati [in August].

"Very sad to not be able to try to help the team play the final."

Spain captain Sergi Bruguera confirmed Albert Ramos had been called up as Nadal's replacement.