Novak Djokovic advanced to the Paris Masters quarterfinals when Damir Dzumhur retired trailing 6-1, 2-1 on Thursday.
Dzumhur had a lengthy massage to his lower back late in the first set and got loud applause when he carried on playing. But after a few more games, he stopped for good. Djokovic was in total control and did not face a break point, breaking his opponent's serve three times.
Djokovic, who will reclaim the No. 1 ranking next week regardless of how he does here, faces Marin Cilic in the quarterfinals.
The fifth-seeded Cilic beat ninth-seeded Grigor Dimitrov 7-6 (5), 6-4, needing six set points to win the first set.
Cilic trailed 5-4 during the tiebreaker but won both points on Dimitrov's serve before serving out the first set.
He started the second set by breaking Dimitrov and was largely in charge after that.
Dimitrov's netted backhand gave Cilic his first match point but Cilic spurned it when he patted a forehand on the run into the net. Another Dimitrov netted forehand gave Cilic a second match point. Dimitrov saved that, too, with a strong serve which Cilic returned long.
Cilic's eighth ace gave him a third match point, and he sealed victory when Dimitrov hit a sloppy forehand wide.
Later Thursday, Roger Federer was facing Fabio Fognini.
Federer is chasing a 100th career title, while Djokovic is looking to win the Paris Masters for a record-extending fifth time.