CINCINNATI - With sidelined Serena Williams about to have a baby, five players have a chance to grab the women's world No 1 ranking at this week's US Open tuneup event in Cincinnati.

Williams, a 23-time Grand Slam champion who announced her pregnancy and the end of her season in April, owned top spot for 186 consecutive weeks through last September, matching Steffi Graf for an Open-era record, and traded the spot with Germany's Angelique Kerber earlier this year.

Karolina Pliskova of the Czech Republic plays a shot against Caroline Wozniak of Denmark (not pictured) during the Rogers Cup tennis tournament at Aviva Centre.  [Photo/Agencies]

Defending Cincinnati champion Karolina Pliskova became No 1 five weeks ago but will lose points from her title run, opening the door for rivals to overtake her.

Romania's second-ranked Simona Halep, world No 3 Kerber, Ukraine's fourth-rated Elina Svitolina and fifth-ranked Dane Caroline Wozniacki could all move to the summit.

"Nothing has changed. I have a big responsibility. I'll do everything as I have before," Pliskova said. "I do think it's a huge thing but I don't want to do anything just because of the rankings.

"I don't try to behave differently. I try not to see the pressure."

Pliskova has won titles this year at Brisbane, Doha and Eastbourne, but anything short of a repeat gives Halep a chance to reach No 1 by winning the title.

"To be No 1 in the world is a big thing. It shows you have been consistent. You have won the place. If I deserve the place, for sure I will win it," Halep said.

However, the Romanian remains haunted by a French Open final loss to Jelena Ostapenko in which she was a set and a break up before squandering a chance at her first Grand Slam title and No 1.

"I still have bad dreams about that," Halep said. "I still suffer."

She's also trying to forget a 6-1, 6-1 loss to Svitolina in the Toronto semifinals, one for which she apologized to spectators on Monday.

"I was really disappointed. It was a horror match for me," Halep said. "I couldn't feel the ball. I can't say I wasn't ready to play. It never happened to me before, not like that."

Svitolina, 22, has won titles this year in Taiwan, Dubai, Istanbul, Rome and Toronto.

She credits Dubai with inspiration but improved in her Canada title run last week.

"The title in Dubai really gave me that push. I played great," Svitolina said. "But this tournament in Toronto showed me I can go there not playing my best and fight back and still be able to win.

"It's one more step in a good direction."

Re-energized

Svitolina's hot season has inspired her to aim big.

"It's very special. It gives me more energy to go for more," said Svitolina, who has never gone beyond the quarterfinals of a Slam.

She needs a title and Pliskova out before the semifinals to reach No 1.

"I know this but I try not to put so much pressure on myself. Of course to be No 1 is a goal of mine. The important thing is to play well," Svitolina said.

"I know it's very close for everyone. I'd prefer to play well at big tournaments. Then the ranking will be there."

Kerber and Wozniacki need Pliskova to crash out in her opening match to have a chance at No 1 this week.

And while Spain's sixth-ranked Garbine Muguruza can't overtake now, she would be a threat to do so at the US Open.

"It's going to be a battle," she said. "I like that I'm part of it and it's going to be a big thing. Now I feel like I've got to play. I can't miss a tournament. If I do I'm gone."

Agence France-presse