NEW YORK -- Heading into this US Open, Ajla Tomljanovic was feeling a bit sad. Serena Williams was retiring and she had never had the opportunity to play the 23-time Grand Slam singles champion. Lying in her hotel bed before the tournament, Australia’s highest-ranked player envisioned herself on the other side of the net from Williams in Arthur Ashe Stadium.

The good news? She was prescient and, yes, sometimes dreams do come true. The bad? She’s playing Serena Williams on Friday night (7 p.m. ET) in Arthur Ashe Stadium.

“She’s paved the way for so many, inspired me to go for my dreams,” the 29-year-old Tomljanovic said after locking up the third-round matchup. “Even her longevity. I’m kind of in the part of my career now where they call you on the older side. She’s made that kind of nonexistent.”

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Indeed, while Williams’ career has been a barrage of unmatched quality, there’s been an enormous dose of quantity, too. She turned professional at the age of 14, some 27 years ago. Twenty-five days from her 41st birthday, she again finds herself on the cusp of history.

A victory over Tomljanovic would make her the oldest woman to reach a Grand Slam Round of 16 in the Open Era that comprises more than a half-century. Williams is only the second female player in the Open Era to win two main-draw matches in a single US Open after turning 40, after Dr. Renee Richards in 1979.

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After losing the second set to No.2 Anett Kontaveit on Wednesday, Williams came back from a brief break “lighter” -- both physically and, perhaps, mentally.

“Quite frankly,” she told reporters, “I just started hitting the ball.”

Said Kontaveit, “She was hitting it a little bit harder, missing a little bit less. She really switched it on from there.”

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This is what the great ones do. They flush away the past and swiftly pivot to the future. Buoyed by a rabidly partisan full house, Williams stroked 12 winners in that ultimate set, balanced by 12 unforced errors. Kontaveit, however, managed only three winners -- and produced 11 unforced errors.

It’s the kind of pressure Tomljanovic will feel as well. One thing she has going for her, though, is some powerful momentum. While Williams’ first and second-round opponents were a combined 1-for-8 in matches coming in, Tomljanovic is playing the best tennis of her life. She made the quarterfinals at Wimbledon for the second straight year (her career best in the majors), and has won 10 of her past 12 matches, going back to Toronto.

Despite the overwhelming crowd support, Williams has been feeling it, too. Aging athletes struggle not only with physical challenges, but also emotional ones. Against Kontaveit, she experienced some understandable moments of weakness under duress. Williams was serving for the first set at 5-3 and was broken.

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Similarly, she was up 2-0 and 40-love in the third when Kontaveit came back to break her. It’s the same vulnerability she demonstrated in the four major finals she’s lost since winning the 2017 Australian Open.

It’s not unexpected when you’re defying -- denying? -- the gravity of age. Roger Federer, who is also 40, has played only 13 matches the past two years, none in 2022. Martina Navratilova played six matches before retiring at the age of 38, losing four of them.

For now, Williams is playing -- to paraphrase the late, great Prince -- like it’s 1999. It’s the secret of her surprising success so far.

“I just feel like I have had a big red X on my back since I won the US Open in ’99,” Williams said Wednesday in her post-match press conference. “It’s been there my entire career, because I won my first Grand Slam early in my career. But here it’s different. I feel like I’ve already won, figuratively, mentally. It’s just pretty awesome the things that I’ve done.”

Not to get too far ahead of ourselves, but things are opening up for her.

Already missing from her bottom half of the draw are the two players slotted in the top and bottom spots: No.2 Kontaveit (courtesy of Serena) and No.7 Simona Halep (qualifier Daria Snigur). Also gone is No.3 Maria Sakkari. If Williams manages to get past Tomljanovic, Liudmila Samsonova -- winner of 12 straight matches -- looms in the Round of 16. The highest seed left in her path, No.5 Ons Jabeur, could be waiting in the quarterfinals.

Tomljanovic will attempt to tune out all the white noise that will surround this highly anticipated match.

“No matter the fact that I’ve been a Serena fan since I was a kid,” she said, “on Friday night I’ll just be a competitor and I’ll try my best to win.”

Far easier said than done -- as Kovinic and Kontaveit discovered.

“I think it’s hard to block out when she’s standing across the net,” Tomljanovic said. “Like, I know who she is. I just remember watching her in the Wimby finals with my sister in front of the TV after my practices. It’s a little surreal to me that I made it, and played in the same era as her.

“I remember Novak [Djokovic] saying one time when they asked him a lot about this, when the crowd was against him, he just pretends it’s for him. When they chant, ‘Rafa, Roger, whoever,’ he hears `Novak, Novak.’ I kind of liked that response. I might use that on Friday night.”