Boris Becker has sensationally questioned whether Nick Kyrgios has been telling the truth about the knee injury that ruined his Australian Open campaign before it even started.

The Australian star announced he was pulling out of his home grand slam on Monday, due to a tear to the meniscus in his knee and a cyst in the joint.

Kyrgios and his physiotherapist, Will Maher, said the injury first became a problem about two weeks beforehand and they used his charity match against Novak Djokovic last Friday night as a test of whether he'd be able to play on through the tournament.

The 27-year-old had a poor preparation for the Open as he was unable to participate in any lead-up tournaments due to an ankle injury he sustained while playing in Dubai.

Becker cast doubt over that version of events, however.

'It didn't look that way on Friday night when he played two sets against Djokovic,' he told Eurosport about Kyrgios' injury battle.

'He talked about injuries beforehand, but to the ankle and not to the knee.

'But what Nick Kyrgios says and what he does are often two different things.'

The pair have feuded in the past, with the Australian lashing out at the three-time Wimbledon champion in 2020.

Kyrgios made a social media post featuring a photo of himself playing with Team World in the Laver Cup, captioned 'different breed' - to which Becker commented, 'In your dreams.'

The Canberran hit back, writing: 'Lingering around like a bad smell, maybe hoping to get more likes and follows. Unfortunately never be as influential.'

He then followed up on Twitter with: 'He’s gone from wanting to coach me, obviously for his desperate need of a pay check, to hating on my Instagram. Strange world we live in.'

They had a consistent online feud in 2020, with Kyrgios making outspoken remarks about tennis stars and their behaviour amid the coronavirus pandemic.

He famously called rival player Alex Zverev 'selfish' after footage emerged of the world number seven breaching social distancing to go dancing in a club in Monte Carlo.

The incident prompted Becker to label Kyrgios a 'rat'.

'Don’t like no #rats ! Anybody telling off fellow sportsman/woman is no friend of mine! Look yourself in the mirror and think your better than us,' Becker tweeted.

'Boris Becker is a bigger doughnut than I thought,' Kyrgios replied. 'Can hit a volley, obviously not the sharpest tool in the shed though.'