Ash Barty reclaims World No.1 ranking in extraordinary US Open twist

Ashleigh Barty is poised to crown her phenomenal season by becoming the first Australian woman to ever top the year-ending tennis rankings.

Barty will officially regain the world No.1 spot on Monday following Naomi Osaka's fourth-round US Open loss to Belinda Bencic.

Osaka, the defending champion at Flushing Meadows, crashed out of the US Open on Monday in a 7-5 6-4 defeat.

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Osaka came into the tournament ranked higher than Barty, but because she won the title in 2018 she had to win it again to defend her rankings points.

Despite both women losing in the fourth round, Barty will now jump ahead.

And the 23-year-old French Open champion is a hot favourite to finish 2019 as No.1 having not only built a healthy rankings lead over her rivals but also having significantly fewer points to defend over the last two months of the season.

Realistically, only Karolina Pliskova can catch Barty, but it will take a huge effort from the Czech to do so.

Barty will enter the Asian swing with a 376-point lead over Pliskova and with 372 rankings points less to defend.

Osaka will slip to No.3 in the world after the US Open and will trail Barty by 1655 points with 45 more points to defend.

That leaves the Japan's reigning Australian Open champion with less than no hope of ending the season as No.1 for a second straight year.

Barty held the top spot for nine weeks after clinching her third title of the season in Birmingham the week before Wimbledon amid a 15-match winning streak highlighted by her grand slam breakthrough in Paris.

Evonne Goolagong Cawley is the only other Australian woman to have the No.1 ranking, for two weeks in 1976.

Barty out to join Lleyton Hewitt

Barty said "it would be incredible" to join Lleyton Hewitt (2001 and 2002) as only the second Aussie to enjoy the year-end top status since women's rankings were introduced in 1975 and men's in 1973.

"I don't know what I would need to do to get there. I haven't really sat down and done the maths or anything like that," Barty said in New York.

"But I have had my schedule planned for the last part of the year for the last few months, so that certainly won't change."

Barty's climb to the summit has been built on consistency.

The Queenslander has won an equal-tour-best 45 wins in 2019 and lost only 11 matches in 12 months since last year's US Open.

Osaka, 21, lost her grip on top spot with a 7-5 6-4 loss to Swiss Belinda Bencic on Monday.

The defeat marked the end of a 17-match winning streak at hardcourt majors.

While Barty and Osaka each won one slam in 2019, Osaka suffered a third-round exit at Roland Garros and was ousted from Wimbledon in round one.

As well as her French Open triumph, Barty was a quarter-finalist in Melbourne and reached the fourth round at both Wimbledon and Flushing Meadows.