Novak Djokovic is into the quarterfinals of the Paris Masters after Damir Dzumhur retired from their match on Thursday.
The Serbian, who will return to world number one next week, was 6-1, 2-1 up when a back injury got the better of his Bosnian opponent.
Djokovic dictated the play from the outset and made the opening break to go 3-1 up, but the match hinged on an 11-minute sixth game.
Dzumhur picked up a back complaint early in the game and called for the trainer, but he continued to fight admirably in a bid to prevent falling to a second break of serve.
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But, the world number 52 appeared to be doing more harm than good, his facial expressions growing ever more strained as he played through the pain.
Djokovic eventually made the break and wrapped up the set to equal his record streak of 29 in succession after Dzumhur elected to play on following a medical time out.
But, the second set did not last long, with Dzumhur calling it quits after Djokovic went a break up at 2-1. Djokovic's 20th consecutive win sets up a last-eight clash with Marin Cilic.