Bulgarian tennis star Grigor Dimitrov has withdrawn from the FAST4 Showdown, reported tennisworldusa.org.

The 27-year-old debuted at the FAST4 event last year. Back then, the Bulgarian beat Australian tennis legend Lleyton Hewitt in a singles match before he and German Alexander Zverev were beaten in a doubles clash by Hewitt and Australian Nick Kyrgios.

However, Team World claimed a 2-1 win over Team Australia despite losing in the doubles clash as the German beat Kyrgios in their singles clash. Dimitrov opened his 2019 campaign at the Brisbane International -- where he made the quarter-final before he was beaten by Japanese Kei Nishikori.

2017 Nitto ATP Finals champion Dimitrov will be replaced by Australian 2018 US Open sensation John Millman at FAST4. The 29-year-old is set to debut at FAST4 as he has no previous appearances at the Australian exhibition.

Millman will join forces with rising star Kyrgios as the two will be eager to help Australia beat Team World -- which will consist of 17-time Grand Slam champion Rafael Nadal and 2016 Wimbledon runner-up Milos Raonic. The FAST4 Showdown is due to be held this Monday at Qudos Bank Arena in Sydney.