Medvedev was trailing Djokovic 7-5, 5-3 when the extraordinary incident brought the ATP 500 semi-final to an abrupt end in Beijing.
Medvedev hits ball into crowd
The incident occurred during a rally with Medvedev under pressure from Djokovic.
After losing the point, Medvedev struck a rebounding ball back towards the other side of the court. The ball travelled into the first-row seating area and struck a spectator in the eye.
The spectator appeared to be in pain and was assisted from the area by stadium staff.
The chair umpire subsequently disqualified Medvedev, who did not contest the decision.
Medvedev apologises
Medvedev expressed his regret after the match and said the incident was completely unintentional.
The former US Open champion said he was particularly concerned about the spectator's condition and hoped to make contact with the person to apologise personally.
Djokovic also expressed sympathy for the injured spectator and Medvedev, having experienced a similar situation himself when he was disqualified from the 2020 US Open after accidentally hitting a line judge with a ball.
Djokovic advances to final
The default handed Djokovic a place in the Beijing final, where he will face Australia's Alex de Minaur.
Djokovic was leading 7-5, 5-3 and serving for the match when Medvedev was disqualified.
The victory extended Djokovic's extraordinary unbeaten record in Beijing to 33 matches. The six-time champion will now have an opportunity to claim another China Open title.
Another interrupted Beijing semi-final
Both men's singles semi-finals ended prematurely on Monday.
Earlier in the day, de Minaur advanced to the final after Hubert Hurkacz retired while trailing 6-4, 3-2. Hurkacz struggled with his movement during the second set before ending the contest.
De Minaur will therefore meet Djokovic in Tuesday's final.
Match summary
• Novak Djokovic: Won by disqualification, 7-5, 5-3
• Daniil Medvedev: Disqualified
Medvedev's unexpected exit ends his Beijing campaign in controversial circumstances, while Djokovic moves into the final without having lost a completed match at the tournament.