Serena Williams' husband has reignited his feud with a billionaire businessman who previously declared his wife was too big and should retire from the sport.

Alexis Ohanian, the co-founder of social news website Reddit, again called out Romanian businessman Ion Tiriac on Twitter after Williams advanced to her 40th career Grand Slam semi-final at the Australian Open on Tuesday night.

It followed a dominant display against world number two Simona Halep, with Williams now just two victories away from equalling Margaret Court's career record of 24 Grand Slam singles titles.

Ohanian again tweeted it was a 'good thing no one listens to that racist sexist (clown) Tiriac,' after the former tennis and ice hockey player infamously stated late last year Williams 'does not move as easily as she did 15 years ago due to her age and weight'.

He then added that 'if she (Williams) had a little decency, she would retire.'

Tiriac was widely condemned for his distasteful comments, but it didn't stop Ohanian again raising the subject on social media.

In response, Tiriac's son Ion Alexandru posted an open letter to Williams' husband on Twitter amid ongoing criticism from fans who questioned Tiriac's claim that he had 'tremendous respect' for the 23-time Grand Slam champion.

'To Mrs. Williams' husband,' it started.

'I read your tweet with a mix of disbelief and disgust.

'The name Țiriac has a 70-year tradition in the world of sports and the gentleman you are referring to has written a few important chapters that have contributed to the book of tennis along these decades.

'From the player, to the coach, to the manager, to the tournament promoter, the respected person you are bad wording in these derogatory terms has and still is, at the age of 82, one of the strongest advocates of this sport.

'The respect he has (Ion Țiriac) for Mrs. Williams is tremendous, as it should be.

'She is and will remain in history as one of the best 20-25 players who have highly contributed to this sport.

'She has a personality that has opened many doors for young generations of athletes, in all fields of sports and in all social walks of life. For that, we are very grateful to her.

'Mr. Williams, the only reason I reply to your unnecessary tweet is that words are misused and misinterpreted in today's world through the various platforms in existence.

'Free speech should not be wrongly replaced with inappropriate slander.'

Ion Alexandru said that Ohanian's words might be seen as casting an 'erroneous image' on Serena's legacy.  

'Congratulations to Serena, very well deserved, keep going forward and writing history in such a wonderful career, Alexandru Tiriac,' the scathing letter finished.

Earlier in the tournament, Ohanian proudly showcased his support for his wife by wearing a t-shirt in the stands which read 'greatest female athlete' with 'female' crossed out.

It left a clear message that Williams is a global sporting pioneer, regardless of gender.

Williams is widely regarded by respected judges as the best women's tennis player of all time. 

In late 2020 when quizzed about Tiriac's comments, she declared it was an 'ignorant comment, and it's a sexist comment, and maybe he is an ignorant man.'

Tenth seed Williams takes on Japan's Naomi Osaka for a place in the final from 2pm (AEST) at Melbourne Park on Thursday.

Williams has won the Australian Open ladies crown seven times, with her first triumph in 2003.