Ashleigh Barty’s world No.1 ranking goes on the line on Thursday when Japanese superstar Naomi Osaka has the chance to reprise her standing as the premier player in women’s tennis.

Barty’s 6-7 (5-7) 6-3 6-4 loss to American Sofia Kenin in her first match of the American hardcourt season on Wednesday has left the door wide open for Osaka to regain the top ranking at the Canadian Open.

The reigning US Open and Australian Open champion, Osaka will return to No.1 if she wins her second-round match in Toronto against German qualifier Tatjana Maria.

Czech former world No.1 Karolina Pliskova can also reclaim the top ranking this week for the first time in almost two years but must reach the semi-finals to have any hope.

The first Australian woman to reach the summit since Evonne Goolagong Calwey in 1976, Barty has held the top ranking for the past eight weeks.

But after enjoying a life-changing 15-match winning streak, the French Open champion’s first successive defeats in almost 10 months have left the 23-year-old in serious danger of relinquishing top spot.

The only way Barty can retain her world No.1 ranking is if Osaka loses her opener in Canada and Pliskova fails to reach the semi-finals.

Osaka is no lay down misere to beat the 74th-ranked Maria, having lost her most recent encounter to the German and also being winless since the French Open.

Openly admitting to suffering in the spotlight of being Japan’s first-ever tennis world No.1, 21-year-old Osaka has fallen to Kazakhstan’s Yulia Putintseva in her past two matches – at Birmingham and Wimbledon.

Pliskova, the first Czech ever to hold the top spot, opens her campaign in Toronto against Alison Riske, the American who snapped Barty’s long winning streak in the Wimbledon fourth round last month.