World number one Novak Djokovic beat Borna Coric to reach the Vienna Open quarter-finals and all but secure the year-end top ranking.
Australian Open champion Djokovic beat Croatia's Coric 7-6 (13-11) 6-3 to reach the last eight in Bulgaria.
Djokovic could only be passed by Rafael Nadal, who would need to take a wildcard into next month's Sofia event to try and overhaul him.
However, if he wins the title in Vienna on Sunday, he cannot be overtaken.
Djokovic will play either Lorenzo Sonego of Italy or Poland's Herbert Hurcakz next.
He saved four set points in a tight opener against US Open quarter-finalist Coric before an early break in the second helped him on his way to victory.