Djokovic advances to Aussie Open semis after Nishikori retires

Novak Djokovic advanced to the Australian Open semifinals when Kei Nishikori retired in the second set after less than an hour of play.

Djokovic was leading 6-1, 4-1 when the eighth-seeded Nishikori let the umpire know he couldn't continue. Nishikori, the 2014 U.S. Open runner-up, played three matches that went to five sets in his first four rounds. Two of those went to the new fifth-set super tiebreaker introduced at the Australian Open this year.

Nishikori needed a medical timeout for treatment on his upper leg after the first set. He wasn't comfortable throughout the match, and had his service broken four times.

Djokovic is aiming for a men's record seventh Australian title. He will next play Lucas Pouille, who beat Milos Raonic earlier Wednesday.