NEW YORK -- Serena Williams left the US Open without her No. 1 world ranking, and without her 23rd Grand Slam championship, too.

While Serena will remain in in New York for Fashion Week and her "Front Row Event," Angelique Kerber is now the US Open champion and the No. 1 player in the world.

"Definitely I would say now that she deserves to be No. 1 because she did so well," said Karolina Pliskova, who upset Serena in the semifinals before falling to Kerber in three sets in Saturday's women's final. "She has two Grand Slam titles, one final. She's been just playing great this year. She deserve to be No. 1.

"And after years [where] Serena was there, I think it's a nice change."

Serena is on the brink of turning 35 and still remains stuck on 22 career Open-Era Grand Slam titles and in a tie with Steffi Graf. Margaret Court is the all-time women's leader with 24 major titles.

Can Serena come back in 2017 and pass Graf, and then Court?

Sure, she can. She's still capable of being the most dominant force in women's tennis for several more years.

But she's also facing a young and hungry group of challengers.

This year, Kerber, no spring chicken at 28, beat Williams in the Australian Open final. Garbine Muguruza, 22, topped Williams in the French Open final. And the big-serving 6-foot-1 Pliskova, 24, took her out in the US Open semis.

"To me, the biggest surprise was Serena had lost the two finals at slams and then a semifinal," said former world No. 1 Martina Navratilova. "Usually in years past she gets more caught out in the first week, not the second week. I think Kerber set it up. Once she beat Serena there, everybody thought, 'Oh, we have a chance.'"

"Maybe it's time changing and different players are coming up," Pliskova said. "Younger players are coming up, which is normal. They are always dangerous. Great to see some other girls winning the title, not only the same person."

Williams certainly didn't think losing her No. 1 ranking made for a nice change.

"I'm not talking about No. 1," she said. "Thank you."

Serena said a left knee injury sustained in the second round of the Open -- that she apparently aggravated during her quarterfinal win over Simona Halep -- hampered her mobility in the semis.

"Yeah, I have been having some serious left knee problems," Williams said.

Injury or not, Williams surrendered her No. 1 ranking and Kerber has no intentions of giving it up.

"I'm ready to have this pressure on my shoulder, because I think I get used to all of this, especially after my first Grand Slam in Australia," said Kerber.

"And to being No. 1, of course now everybody will try to beat me and have nothing to lose. I will try to take this challenge, because it will be a little bit new situation for me. But at the end, I was always practicing and working hard to being No. 1."

Pliskova may be happy to see Kerber replace Serena atop the rankings, but not everybody thinks we've seen the last of Serena winning major titles.

"I don't think Serena is going anywhere," ESPN's Mary Jo Fernandez said. "I think she's going to win more and break Steffi's record, but it's going to get tougher. It gets harder the older you get and the young ones coming up."

Follow Adam  Zagoria on Twitter @AdamZagoria for updates throughout the U.S. Open.