Without a win at Melbourne Park in the past five years, the Australian No.2 dismantled the 31st seed 6-1 6-1 in an hour and 17 minutes on Tuesday.

Tomljanovic, no stranger to an upset after beating then-world No.3 Agnieszka Radwanska at the 2014 French Open, put the win among her greatest grand slam triumphs.

"This is one of my best wins quality-wise in a slam first round," she said.

"(It) felt really good to be out there. I think from the first point, I felt ready. Felt like I was hitting the ball clean."

Despite only dropping two games on her way to the win, the world No.52 felt the scoreline slightly flattered her performance.

"Honestly, it felt way, way closer ... especially in the second set. Those two-love games and I think 3-1 were very important.

"I was gassing a little bit at two-love. I was really happy to close it out."

The Croatian-born Australian reached a career-high singles ranking of world No.39 in March, 2019 before dropping back outside the top 50.

She capped the year by making her Fed Cup debut late last year as Australia fell 3-2 in the final to France.

Tomljanovic was relieved to knock off Sevastova after marathon three-set losses to the Latvian and Brit Johanna Konta in the past two years at Melbourne Park.

"I think I've had a couple of tough draws, real heartbreaker last year against Jo," she said.

"Coming in, I still thought this was a tough draw. Actually had her two years ago at six-love, three-love up (and) ended up losing.

"I know she never goes away. She's so tricky."

But break-point chances told the story of the match, with Tomljanovic converting five of 13 to Sevastova's none from seven.

The 26-year-old joins top seed Ashleigh Barty as the second Australian woman to book a place in the round of 64.

But there will be another challenge awaiting her in the second round in the form of French Open and Wimbledon champion Garbine Muguruza, who fought back to win 0-6 6-1 6-0 over American Shelby Rogers.

Australian Associated Press