Novak Djokovic all-but guaranteed the year-end number one spot for the sixth time in his career, equalling the record held by Pete Sampras, after battling into the Erste Bank Open quarter-finals.
Only Rafael Nadal can now overhaul Djokovic but it would involve the Spaniard taking a wild card as a late entry into the Sofia Open in Bulgaria next month.
If Djokovic lifts the title in Vienna, he cannot be surpassed.
The Serbian took another stride towards doing that with a hard-fought win over Borna Coric in the second round, with the Croatian spurning four set points in a tense tiebreak before going down 7-6 (11) 6-3.
“It was very tiring and very challenging,” Djokovic said in quotes on the ATP’s website. “Obviously Borna is a great fighter.
“He’s a very good friend of mine. Off the court we’ve known each other for quite a long time.
“I’m just really glad to stay focussed and play my best at the right time.”
The top seed will next take on either Poland’s Hubert Hurkacz or Italian lucky loser Lorenzo Sonego.
Kevin Anderson was the only other player to reach the last eight after the South African big-hitter defeated Spain’s Pablo Carreno Busta 7-5 6-1.
In the first round, third seed Stefanos Tsitsipas was tested to his limits before seeing off Germany’s Jan-Lennard Struff, with the Greek on court for nearly two hours before securing a 6-7 (3) 6-3 6-4 win.
Fourth seed Daniil Medvedev needed just 66 minutes to thrash Jason Jung, with the Chinese Taipei player bowing out after a 6-3 6-1 defeat.
The Russian will face Vasek Pospisil next after the Canadian qualifier overcame compatriot Felix Auger-Aliassime 7-5 7-5, while Italian wild card Jannik Sinner saw off Casper Ruud 7-6 (2) 6-3.
At the Astana Open in Nur-Sultan, Kazakhstan, second seed Miomir Kecmanovic was sent packing after the Serbian was beaten by America’s Francis Tiafoe 7-5 6-3.
There were no such concerns for John Millman, seeded fourth, as the Australian comfortably beat Spanish veteran Fernando Verdasco 6-3 6-4.
Home hope Mikhail Kukushkin hit back to oust Argentina’s Federico Delbonis 6-7 (5) 6-3 6-2 but fellow Kazakh and sixth seed Alexander Bublik was hammered 6-3 6-2 by America’s Mackenzie McDonald.
Seventh seed Tommy Paul was relatively untroubled as the American dispatched Romania’s Radu Albot 6-4 6-0, while Egor Gerasimov of Belarus overturned a one-set deficit to prevail 4-6 6-3 6-4 over Jiri Vesely of the Czech Republic.