Millman replaces Dimitrov in Fast4 tennis

John Millman has replaced Grigor Dimitrov in the Fast4 exhibition tennis event in Sydney.

Bulgaria's Grigor Dimitrov is out of the Fast4 tennis event in Sydney on Monday night. (AAP)

Former world No.3 Grigor Dimitrov has withdrawn from the Fast4 exhibition in Sydney and has been replaced by Australian John Millman.

The world No.38 Millman will play alongside Nick Kyrgios on Monday night in a match-up against Rafael Nadal and Milos Raonic at Sydney's Qudos Bank Arena.

Dimitrov is already in Melbourne ahead of the Australian Open, having completed a hitting session with Serena Williams on Sunday morning just before withdrawing in Sydney.