Australian underdog Ajla Tomljanović has fallen just short of a massive upset of world No.2 Simona Halep at the Australian Open.

Tomljanovic, 27, took a 5-2 lead in the third and served for the match but the reigning Wimbledon champion steamed back to win five games in a row to secure a 4-6 6-4 7-5 victory.

The 72nd-ranked Tomljanovic used her big groundstrokes to pummel the 29-year-old Romanian, who was an Open semi-finalist last year and finalist in 2018, but fell agonisingly short. 

The pair duked it out at Melbourne Park's Margaret Court Arena on Wednesday with the Croatian-born Aussie taking the first set off the two-time Grand Slam winner.

But the gritty former world number one fought her way back to claim the second set.

With the stage set for a thrilling finish, it appeared Tomljanovic was going to pull off an incredible upset.

She was two games up in the third, and then won another tie break to bring the match to 5-2. 

But the experience and steely nerves of Halep eventually prevailed as she served out the final set 7-5 after surviving a match point.

The relief on Halep's face was evident and she said after the match she'd expected a tough test in the second round. 

'I expected that she's going to play very hard and very strong,' Halep said. 

'I know that she's a very tough opponent and I expected it to be a difficult match, but it was more than I thought.  

'I was not that positive when I was talking to myself. Mentally, I think I was a little bit stronger and I wanted to, I had the desire to win the match and I didn't want to give up.'

Tomljanovic - who is dating men's world No.10 Matteo Berrettini - made it to the second round of the tournament last year and was hoping to go deep into her adopted home's Grand Slam this time around. 

After Covid interrupted last year's travel scheduled, she said she now has a different outlook on her career.   

'I cherish it more, but at the same time I still have that pressure that I put on myself,'  Tomljanovic told Tennis Australia in an interview before the match.

'I think before – I wouldn't say I took it for granted, but I took it more as a kid,' she admitted.

'Now I feel not that my time is limited, but I want to treat every tournament as a huge opportunity that can take me places.

'I definitely do think I'm a little bit not on the older side, but just a little more experienced and mature. I'm not 20 anymore.'