Novak Djokovic virtually ensured equalling Pete Sampras's record of ending the year as world number one for the sixth time on Wednesday when he reached the Vienna ATP quarter-finals.

Djokovic defeated Croatia's Borna Coric 7-6 (13/11), 6-3 and will secure the year-end top spot as long as Rafael Nadal does not take a wild card into the Sofia event next month.

If the 33-year-old Serb wins the Vienna title on Sunday, he will make sure of the landmark regardless of whether or not Nadal plays in Bulgaria.