BEIJING - Six-time China Open champion Novak Djokovic extended his tournament winning streak to 33 matches and entered the men's singles final here on Monday, while home favorite Zheng Qinwen beat Marie Bouzkova of the Czech Republic to reach the women's singles round of 16.

In the men's singles semifinals, Djokovic advanced after third-seeded Daniil Medvedev was defaulted for accidentally hitting a spectator with a ball while trailing 7-5, 5-3. The Serbian will face fifth seed Alex de Minaur on Tuesday after the Australian advanced when Poland's Hubert Hurkacz retired with an adductor injury while trailing 6-4, 3-2.

"It happened by accident. I am sorry for Daniil and for the audience," reflected Djokovic after the match. "You know, it also happened to me six years ago in US Open. I know it must feel bad."

ALSO READ: China Open: Djokovic rallies past Zverev for record 32nd win

"I mean, 100 percent accident. You can see that I didn't hit the ball in anger," said Medvedev. "When I saw the fault, I felt horrible."

"Terribly sorry to the spectator. I'm trying to get in touch with him, if he wants it. Hopefully I can apologize in person and see how he feels," he added.

Olympic champion Zheng Qinwen defeated 18th seed Marie Bouzkova 7-6 (3), 4-6, 7-5 to set up a round-of-16 meeting with Armenia's Alina Charaeva, who eased past Britain's Sonay Kartal 6-4, 6-1.

US Open girls' singles champion Sun Xinran fell 7-5, 6-1 to world No. 4 Coco Gauff. Third-seeded Gauff will meet 13th seed Elise Mertens, who beat Latvia's Jelena Ostapenko 6-2, 3-6, 6-3.

In other matches, eighth seed Iga Swiatek came from behind to beat Donna Vekic 4-6, 7-6 (0), 6-2 and remain in contention for a second China Open women's singles title.

This year's French Open champion Mirra Andreeva will face Ukraine's Daria Snigur on Tuesday, while Japan's Naomi Osaka will take on Karolina Muchova of the Czech Republic.