VIENNA -- 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, 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 with aces but hit a backhand long on his opponent's 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 didn't 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 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.