Serena Williams broke into tears after she lost in the semifinals of the Australian Open in Melbourne to Naomi Osaka

Serena Williams broke down in tears before cutting short her post-match news conference after losing 6-3 6-4 to Naomi Osaka in the Australian Open semi-finals on Thursday.

The defeat ended Williams' bid for a record-tying 24th Grand Slam title.

'It was a big error day for me today,' the 39-year-old told reporters after the match at Rod Laver Arena.

'I had so many opportunities, ... it was just ... I made too many mistakes there and easy mistakes,' said the American. 

'Not like .... I was on the run or anything. They were just easy, easy mistakes.' 

Asked if it was just a bad day at the office, Williams said: 'I don't know. I'm done,' before leaving the room in tears.