Novak Djokovic kept his perfect record at the China Open alive on Friday, rallying past home favourite Bu Yunchaokete 4-6, 7-6(2), 6-2 in Beijing. The win took two hours and 52 minutes.

The 39-year-old Serb is now 31-0 at the tournament, where he has won all six of his appearances. His titles came in 2009, 2010 and from 2012 to 2015. This is his first appearance in Beijing since 2015.

Djokovic needed medical timeouts during the match for his right shoulder and abdominal area. After the win, he admitted this year has been a struggle with physical issues he does not want to disclose. “I don’t want to go into details because then it’s going to sound like I’m whining,” he said.

His 31-0 start at one event matches Rafael Nadal’s 31-0 mark at the French Open, the best start at any tournament by a man in the Open Era.

In the quarterfinals, Djokovic will face No. 1 seed Alexander Zverev. The German beat China’s Juncheng Shang 6-0, 6-3 in just 55 minutes, striking nine aces without facing a break point.