World No. 1 Aryna Sabalenka heads into this week's WTA 1000 tournament in Toronto with her top ranking under threat as she builds toward her U.S. Open title defense.

Sabalenka has won 23 of her 24 hard court matches this year, the lone defeat her loss to Elena Rybakina in the Australian Open final.

But she was turfed out in the quarterfinals on the clay of Roland Garros and fell to Naomi Osaka in the Round of 16 on the grass courts of Wimbledon.

If Sabalenka fails to reach the Round of 16 in Toronto, Kazakhstan's Rybakina can rise to No. 1 if she lifts the trophy.

Sabalenka has never made it past the semifinals in Canada, which alternates each year between Montreal and Toronto, but she said she was feeling rested and ready to reassert herself after taking time off in the wake of her All England Club exit.

"Clay and grass, I wouldn't say they were that bad," she said. "I was trying a couple of things, didn't go the way that I expected. And now we're kind of back to basics and I really hope it's going to work out well."

Sabalenka had exited Wimbledon saying she wanted to "forget about tennis."

She did that with a summer holiday on the Greek island of Mykonos.

"I think the rest was much-needed," she told reporters Sunday. "I spent there five, six days maybe. I felt much better, I couldn't wait to get back on court and start working toward my goals for this swing."

Sabalenka heads a top half of the draw that also includes two-time winner Jessica Pegula, the third seed, 2024 finalist Amanda Anisimova and seventh-seeded Iga Swiatek of Poland.

Former world No. 1 Swiatek, a six-time Grand Slam champion, will be aiming to regroup after slumping to a third-round defeat at Wimbledon.

Rybakina anchors a bottom half of the draw that also includes fourth-seeded Coco Gauff, four-time Grand Slam champion Osaka and newly crowned Wimbledon champion Linda Noskova.

Czech Noskova, who broke into the Top 10 after winning her first Grand Slam title at Wimbledon, is seeded sixth and could face another rising star in the Philippines' Alexandra Eala in the third round.

They have met twice already this year, with Noskova winning both times.

Wimbledon runner-up Karolina Muchova is among the notable absentees as she recovers from a minor surgery.

Reigning champion Victoria Mboko of Canada, who thrilled Montreal fans with her run to a first WTA title last year, remains sidelined with the knee injury that forced her out of Wimbledon.

The ATP Masters 1000 in Montreal this week has also been hit by high-profile defections, including that of Wimbledon champion Jannik Sinner.

Sabalenka said the scheduling is "tricky" for any player that does well in the grass-court season, with the Canadian tournaments followed by a 1000-level event in Cincinnati in a schedule that offers little respite before the U.S. Open begins on Aug. 30.

"If you go far at the grass court (season) you feel really fatigued and you're not able to pull out any good tennis so what's the reason of coming when your body is really so much stress," she said. "It's very tricky, but this year I didn't do well so I'm here."

All 32 seeded players received first-round byes.

Play got underway Sunday with Japanese qualifier Moyuka Uchijima booking a second-round meeting with Sabalenka with a 6-3, 6-4 victory over Spain's Cristina Bucsa.

Czech Sara Bejlek posted a 6-3, 4-6, 7-5 victory over China's Wang Xiyu, who was hobbled by a left leg problem, and booked a second-round meeting with Swiatek.

Seven-time Grand Slam champion Venus Williams bowed out in the first round, falling 6-4, 6-1 to Uzbekistan's Kamilla Rakhimova.