Kei's withdrawal sends the defending champion into last 4 where he will face Rafael Nadal (ESP, 4).
Novak recorded a 6-4, 7-5 victory over Feliciano Lopez in one hour and 33 minutes on Thursday.
Nole goes on to play Feliciano Lopez (ESP, H2H 8-1) in the last 16 on Thursday.
Nole will next play against no.10 seed David Goffin, who recorded a 7-6, 4-6, 6-3 win over sixth seed Dominic Thiem earlier in the day.
Next up for the two-time former champion is no.13 seed Pablo Carreno Busta (ESP).
Djokovic earned his 300th ATP World Tour Masters 1000 victory, beating no.31 seed Juan Martin del Potro 7-5, 4-6, 6-1.