Novak Djokovic saved four set points in a 7-6 (11), 6-3 win over Borna Coric on Wednesday to reach the Erste Bank Open quarterfinals and move one step closer to sealing the year-end No. 1 ranking.
Djokovic can lock up the top position by winning the title on Sunday.
Otherwise Rafael Nadal could still mathematically overtake him, although the Spaniard would need to enter a tournament in Bulgaria next month in order to do so.
Djokovic will play either Hubert Hurkacz or Lorenzo Sonego in Fridays quarterfinal.
Earlier Wednesday, third-seeded Stefanos Tsitsipas and No. 4 Daniil Medvedev won their first-round matches.
Djokovic, who had not lost a set in three previous matches against Coric, saved two break points at 4-4 and one more in his next service game at 5-5.
Coric missed four set points in the 18-minute tiebreaker, including one on his own serve when he hit a forehand wide.
The Croatian saved two set points for Djokovic with aces but hit a backhand long on his opponents third chance.
Borna is a great fighter. He has kind of similar style of tennis as I do. In the backhand corner very solid, trying to create points with the forehand, Djokovic said in an on-court interview.
He was serving very well, especially in the first set. It was very difficult for me to return his serve, said Djokovic, who also held off a set point in his first-round match.
The tiebreak again, like yesterday against (Filip) Krajinovic, decided the match, I guess, he said.
Djokovic was in control of the second set after Coric double-faulted when facing a break point at 1-1.
Djokovic didnt allow Coric another chance on his serve and closed out the win on his second match point.
I am just really glad to stay focused and play my best at the right time, Djokovic said.
Tsitsipas rallied to defeat Jan-Lennard Struff 6-7 (3), 6-3, 6-4 and set up a second-round match with Grigor Dimitrov.
Tsitsipas held serve throughout but still lost the opening set in a tiebreaker before improving to 3-2 against his German opponent.
Medvedev advanced to the second round with a 6-3, 6-1 win over Jason Lung, who replaced the injured Alex De Minaur.
Medvedev next plays Vasek Pospisil, who beat Canadian countryman Felix Auger-Aliassime 7-5, 7-5.
Also, Jannik Sinner defeated Caspar Ruud 7-6 (2), 6-3 to set up a match against Andrey Rublev.