Second seed Novak Djokovic reached the quarter-finals of the Paris Masters after Bosnia's Damir Dzumhur retired with a back injury.
The Serb, 31, who will take the world number one ranking from Rafael Nadal on Monday, was leading 6-1 2-1 when Dzumhur ended the match.
The Wimbledon and US Open champion did not face a break point in the match and won 72% of first-serve points.
He will now face Marin Cilic after the Croat beat Bulgaria's Grigor Dimitrov.
Cilic hit 29 winners as he claimed a 7-6 (7-5) 6-4 victory over ninth seed Dimitrov.