Serena Williams at No. 1, despite not playing since January

Serena Williams is briefly back at No. 1 in the WTA rankings, despite not playing a match since January - and with plans to take the rest of 2017 off because she is expecting a baby.

Thanks to a calendar quirk, Williams moved up one spot Monday from No. 2, swapping places with Angelique Kerber.

The return to No. 1 , which gives the 35-year-old American her 317th week there, comes less than a week after Williams let the world know via Snapchat that she is pregnant. The baby is due in the fall, and spokeswoman Kelly Bush Novak said Williams will take off the rest of this season and intends to return to the tour next year.

Williams confirmed that timetable on Instagram on Monday, when she posted a photo of herself and a note addressed to ''My Dearest Baby.''

''You gave me the strength I didn't know I had. You taught me the true meaning of serenity and peace. I can't wait to meet you. I can't wait for you to join the players box next year. But most importantly, I am so happy to share being number one in the world with you.... once again today,'' the post reads.

Williams closed it with: ''from the world's oldest number one to the world's youngest number one. -Your Mommy''

The reason Williams was able to supplant Kerber for at least a week - and possibly more - is that the women's clay-court tournament that is being played this week in Stuttgart, Germany, was played a week earlier a year ago. So Kerber, a two-time defending champion at that event, lost the rankings points she earned there and can't replace them until the end of this year's tournament.

Kerber can reclaim the top ranking by winning two matches in Stuttgart.

Williams hasn't competed since winning her Open era-record 23rd Grand Slam singles title at the Australian Open at the end of January. She was pregnant at the time.

---

Follow Howard Fendrich on Twitter at http://twitter.com/HowardFendrich

---

More AP tennis: https://apnews.com/tag/apf-Tennis

SI Apps
We've Got Apps Too
Get expert analysis, unrivaled access, and the award-winning storytelling only SI can provide—from Peter King, Tom Verducci, Lee Jenkins, Andy Staples, Grant Wahl, and more—delivered straight to you, along with up-to-the-minute news and live scores.