Tomljanovic, who upset Serena Williams on the way to making the US Open quarter-finals two years ago, never found her footing in a 6-3, 6-2 thrashing at No.33 seed Elise Mertens’ hands.

Mertens made her move in the seventh game after both players held serve until then.

The Belgian built pressure with an inside-out backhand winner before Tomljanovic’s risky down-the-line backhand narrowly missed to bring up double break point.

A double fault – one of the Australian’s 10 for the match – handed Mertens the break, and she never looked back.

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American-born Joint, who switched allegiances to her Melbourne-raised father Michael’s birthplace last year, qualified for the US Open then upset German veteran Laura Siegemund to earn an Arthur Ashe Stadium shot at Madison Keys.

Joint gave a good account of herself in the first set before 2017 finalist Keys’ heavy groundstrokes began to take a toll in a 6-4, 6-0 defeat in an hour and two minutes.

“It was a competitive first set, and I had my chances, but she played really well,” Joint said. “She lifted in the second set for sure, and I don’t think I did anything wrong.”

The teenage rising star’s backhand, in particular, held up against the 14th-seeded American, and she won more than 80 per cent of first-serve points in the opening set – but Keys punished Joint’s second serve.

That will be an area for Joint to work on, but the breakthrough performance at her maiden grand slam could see her jump as many as 27 spots to a career-high ranking of No.108.

She still plans to start a psychology degree and play tennis at the University of Texas after the US Open, but is on track to earn direct entry into next year’s Australian Open.

In other results, Wimbledon champion Barbora Krejcikova suffered a shock 6-4, 7-5 loss to 122nd-ranked Romanian qualifier Elena-Gabriela Ruse.

The other seed to fall was Argentine Sebastien Baez, who retired with a foot injury while trailing Tallon Griekspoor 6-1, 2-0.

No.10 seed Alex de Minaur, Stefanos Tsitsipas’ conqueror Thanasi Kokkinakis, Jordan Thompson, Chris O’Connell, wildcard Tristan Schoolkate and Max Purcell will play their second-round matches on Friday (AEST).

Watch every match of the 2024 US Open ad-free, live and on demand on Stan Sport with select matches live on 9GO and streaming on 9Now.