Newly-crowned US Open champion Stan Wawrinka has dealt a blow to next week's Japan Open after he was forced to withdraw due to a back injury.



It is the second shock to Japan's showcase ATP event after Olympic silver medallist Juan Del Potro announced last week that he would not be playing.



The absences clear the way for national tennis hero Kei Nishikori to bid for his third home title.



The world No.5 won the trophy at the Ariake Colosseum in 2011 and 2014. Wawrinka played last week in St Petersburg, losing the final to German teenager Alexander Zverev, who then withdrew from a tournament in China this week due to illness.

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Wawrinka won the Tokyo title a year ago over Frenchman Benoit Paire. The Swiss is due to play from October 9 at the Shanghai Masters, which is already missing world No.1 Novak Djokovic due to an elbow injury.