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World number one Rafael Nadal has withdrawn from the Paris Masters with a knee injury.

The Spaniard was due to face Serb Filip Krajinovic in the quarter-finals on Friday, but pulled out with a right knee problem before the match.

Nadal had the knee strapped during his third-round win over Pablo Cuevas.

Juan Martin del Potro missed out on securing the final qualifying spot for the ATP World Tour Finals as he was beaten 6-4 6-7 (5-7) 6-4 by John Isner.

Nadal said he would "do his best" to return for the season-ending event in London, which takes place from 12-19 November.

"For me it's not about London. For me it's about the longer term," he added.

"And longer term, I'm going to do the treatment I believe that is going to be more safe and more important to keep playing tennis as long as possible."

Argentine Del Potro had to win to feature in London, but American Isner ensured he stayed in contention for the World Tour Finals with a hard-fought victory in two hours and 12 minutes.

Either Isner, compatriot Jack Sock and Spain's Pablo Carreno Busta will take the final qualifying spot.

However, Del Potro will return to the top 10 in the world rankings on Monday for the first time since July 2014 at the expense of Novak Djokovic.

The Serb 12-time Grand Slam champion will drop out of the top 10 for the first time since 2007.