When she returned in 2016, she hobbled through a fourth-round defeat against Zhang Shuai and was in tears in the players’ lounge afterward, struggling to articulate her disappointment.

Last year, she was unable to make the long journey to Melbourne at all, with her left wrist in a cast after surgery. Following the matches from afar was difficult.

“I watched a handful, but it made me really sad, so I pretty much avoided , Tennis Channel, all of it,” she said Monday as she answered questions on court in the Australian sunshine.

But her mood and her power tennis are back on the upswing here, and if there were any lingering doubts about the state of Keys’ game, she dispelled them in a hurry in Rod Laver Arena by overwhelming the eighth-seeded Caroline Garcia, 6-3, 6-2.

In the quarterfinals, Keys will face Angelique Kerber, the former No. 1-ranked player and the 2016 Australian Open champion, in what should be a compelling contrast in styles: Keys’ punching power against Kerber’s counterpunching athleticism.

Kerber, seeded 21st but undefeated this season after winning the title in Sydney, was close to a surprise defeat Monday before prevailing, 4-6, 7-5, 6-2, against Hsieh Su-wei, the 88th-ranked Taiwanese veteran with an unconventional game.

Hsieh, 32, had upset No. 3-seeded Garbiñe Muguruza and No. 26-seeded Agnieszka Radwanska with her clever tactics, great defense, abrupt changes of rhythm and tightly angled shots struck double-handed off both sides.

She was one game away from bamboozling Kerber, too, but after Hsieh took a 6-4, 5-4 lead, Kerber held serve to 5-5 and got the better of a series of grueling, extended exchanges to take the set.

“I was running everywhere and she always had the answer,” Kerber said on the court after closing out her ninth straight victory (13 if you count the Hopman Cup team event). “We will see a lot in 2018 from her, that’s for sure.”

That could also be true of Garcia, a 24-year-old from France who propelled herself into the top 10 late last season with a series of attack-minded victories in Asia, winning two tournaments in China and reaching the semifinals of the WTA’s year-end championships in Singapore.

A back problem forced her to retire during her first tournament of this year in Brisbane, Australia, but Garcia looked fit and confident heading into Monday’s duel. She opened the match by stepping inside the baseline to return aggressively, breaking Keys’ intimidating serve in the first game.

But the 17th-seeded Keys bounced back to break Garcia, and then proceeded to play one of her finest matches in a major tournament.

Keys, 22, reached her first major final at the U.S. Open in September but struggled to handle the moment, making errors in bunches and losing, 6-3, 6-0, to her compatriot and friend Sloane Stephens.

“Pretty much the week to 10 days after was really tough,” Keys said. “I mean, my sleep schedule was totally messed up. I was just really tired from all the late, emotional matches that I had. And then to have such a great two weeks and then have it end the way that it did, it was really devastating for me. So it definitely took some time to get over.”

Stephens has yet to win another match since that victory, and Keys played only one more match in 2017, losing to Varvara Lepchenko in the opening round in Wuhan, China, before deciding to end her season early to focus on getting her postoperative wrist in ideal shape to start 2018.

But she also used her relatively long offseason to work on her conditioning, and it showed in the form of improved footwork, lateral speed and quickness against Garcia, who repeatedly tried to attack her traditional weaknesses without success.

“There’s a sharpness about her,” said Sam Smith, the former British player, who is a television analyst for Seven Network in Australia. “She’s getting to the ball a fraction faster, and the setup is more precise. Hence she is making fewer unforced errors.”

Keys still made 21 on Monday, but she also hit 32 winners, including nine aces, and managed to win the majority of the rallies, no matter what the length of the exchange.

She said she was playing “really smart,” but when asked if this was a new step for her, her answer was polite but clear.

“I don’t think so,” she said smiling. “I think I played pretty smart to get to the finals of the U.S. Open and win some titles. Hopefully it’s just a little bit more consistently that way.”

Keys has had world-class power from the baseline since her midteens. It comes naturally, and it can be a double-edged sword as her flat strokes often leave her little margin for error.

But Smith believes that Keys, under the guidance of her coach, Lindsay Davenport, is able to hit with more topspin on the backhand side now, creating more clearance when Keys needs it.

“Each shot used to be an individual event,” Smith said. “But Lindsay Davenport has taught her how to play tennis.”

Keys, identified long ago as a potential champion, has had no shortage of guidance during her career. Her ability to control her nerves as well as her power will be decisive as she chases her first Grand Slam title.

Bigger challenges await in the second week here, but she has yet to lose a set in four matches, or be pushed to a tiebreaker.

“I’m really excited,” said Keys, the last American woman in the singles tournament. “I think I’m playing well and especially in tight moments, handling my emotions well.”

In the men’s draw, Tomas Berdych reached the quarterfinals with a 6-1, 6-4, 6-4 victory over Fabio Fognini. He will play the winner of the match later Monday between Roger Federer and Marton Fucsovics.

The New York Times