MELBOURNE, Australia — Serena Williams became a time traveler on Sunday, pulled back to the past to essentially face down her much younger self.

Across the net from her in the fourth round of the Australian Open stood the 22-year-old Belarusian Aryna Sabalenka, who turned pro at 14, like Williams, and whose strategy called to mind Williams’s game plan at the same age: If at first you don’t succeed, hit harder.

Williams, 39, stared down Sabalenka, and after two gripping hours, Sabalenka blinked. In the 10th game of the deciding set, Sabalenka mustered one point on her serve as Williams, a seven-time champion, seized the break and a 6-4, 2-6, 6-4 victory to set up a quarterfinal meeting with Simona Halep, who dispatched the 19-year-old Iga Swiatek in three sets.

Williams’s longevity makes it easy to forget that before she was the game’s grande dame, she was its whiz kid, collecting nine WTA singles titles, including one Grand Slam, before she was out of her teens.

Sabalenka, a nine-time winner on the WTA Tour, and Swiatek, the reigning French Open champion, are the latest in a long string of polished phenoms threaded through Williams’s career. One of the biggest stars to emerge, Naomi Osaka, saved two match points to beat Garbiñe Muguruza on Sunday. Still, from Jennifer Capriati and Monica Seles to Maria Sharapova and Sloane Stephens, Williams has watched many young talents come and go and, on occasion, stray far from tennis.

A sport with a history of suffocating its young has not stifled Williams, a 23-time Grand Slam champion in singles whose love for the game seems to have deepened over time. Against Sabalenka, she studied a page of written notes during changeovers as if she were back in high school. She fiddled with her “Queen” necklace. She dug balls out of the corners and ran from side to side as if she were on a school blacktop at recess.

Darren Cahill, one of Halep’s coaches, described Williams’s movement as the best he had seen from her “in a long, long time” and said, “If you can stay in more points and get more balls back, stay alive, then she’s got the power to turn those points around.”

What Williams is doing is also inconceivable to the younger Americans, three of whom have followed her into the second week. Marveled, one of the three, the 28-year-old Shelby Rogers: “What she’s been able to accomplish is absolutely incredible because some days I wake up now and I’m like, ‘OK, I’m not 21 anymore.’”

Williams’s serve usually allows her to win her share of easy points. But against Sabalenka, her main weapon continually misfired. Williams put 52 percent of her first serves in play and recorded eight double faults, including one in the fifth game of the third set, which gave Sabalenka two break points.

With the state of Victoria in Day 2 of a hard lockdown, no fans were in the stands, but the restrictions placed on the local populace did not extend to Williams’s inner circle, which includes her husband, coach, agent, hitting partner and older sister Venus, 40, who lost in the second round.

Williams didn’t need to be told by the Victorian premier, Daniel Andrews, that her entourage qualified as “essential workers,” a classification that made it possible for them to attend the match. Her team is elemental to her success, and she looked over often to where everyone was seated. When she was down 15-40 in that fifth game, Venus raised both hands as if signaling a touchdown and they locked eyes.

Williams’s most recent Grand Slam championship came at Venus’s expense at Melbourne Park in 2017, when she was two months pregnant with her first daughter, Alexis Olympia Ohanian. Since becoming a parent, Williams has found her voice as an advocate for working mothers everywhere, speaking openly of the hardships, both physical and emotional, that she and others on the WTA Tour — and in the wider world — confront daily while balancing their jobs and child-rearing.

But in that telepathic moment between the sisters, Serena was not tennis’s earth mother. She was transported back in time to her early years as a pro when she looked to Venus for direction.

“When I hear her voice, it just makes me calm and confident,” Williams said. “Yeah, I think there’s something about it that just makes me feel really good.”

She got her first serve in on the next three points and won them all, earning an advantage with a 126 mile-an-hour ace. Williams closed out the game on a frazzled Sabalenka’s forced error.

Sabalenka fought back, winning the next three games to draw even at 4-4. At that point, she said: “I felt like I should win it. I felt like I was fighting really well.”

But so was Williams. She held, and with Sabalenka serving to stay in the match, Williams got enough balls back to fluster her younger opponent, whose service game ended with a double fault and two forehand unforced errors.

“I just needed to play better on the big points,” Williams said. “I knew that I could. I still hadn’t reached my peak. I was like, ‘OK, Serena, you got this. Just keep going.’”

After 23 major singles titles and hundreds of millions of dollars in prize money and endorsements and motherhood, how does Williams find the motivation to keep chasing a tennis ball?

The answer could be found in how Williams spent her off day. After her Saturday practice, she put her daughter down for a nap and then made work calls to the United States, finalizing orders and obsessing about fabrics for her fashion line, S by Serena, which she described as her “second career.”

There’s a method to Williams’s multitasking. She has been doing it her whole life, she said. She never played a full tennis schedule as a junior and has never played a full schedule as a pro.

“I still went to college, I still did a lot of other things,” Williams said. “I had other careers. It was impossible to burn out.”

Convention holds that Williams continues to play because she has Margaret Court’s career record of 24 Grand Slam singles titles in her sights. But the truth might be simpler.

“I like my job,” she said. “I like what I do. It’s pretty special I get to come out and still get to do it.”