Naomi Osaka advanced to the third round of the Rogers Cup in Toronto, meaning Ash Barty will lose her spot at the top of the WTA rankings.
Barty suffered a surprise defeat to Sofia Kenin in her first match back after Wimbledon in July.
After beating Kenin three times already at Tour level, the American was too good this week, taking a 6-7(5) 6-3 6-4 victory.
The result meant that either Osaka or Karolina Pliskova had the opportunity to wrest back the number one spot in the rankings by the end of the week.
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The only way Barty could reclaim number one position was if Osaka was knocked out in her first match and if Pliskova didn't advance to the semis.
As it stands, Osaka is in pole position to take back top spot, which Barty took from her eight weeks ago.
Osaka took the first set 6-2 against Tatjana Maria in Toronto, before the German retired hurt with an abdomincal injury.
Pliskova needs to make the semi finals to have any hope of taking the number one ranking, which she held in 2017.
If they each advance that far, Osaka and Pliskova could play each other in the final to crown the new world number one.
Osaka went to world number one in January after claiming her debut Australian Open win with a victory over Petra Kvitova.
It's a position she maintained until June, when Barty dethroned her with success at the French Open and in Birmingham.
Barty was knocked out of Wimbledon in the fourth round by Alison Riske, while Osaka suffered a shock first round defeat to Yulia Putintseva.
Osaka will now play either Caroline Wozniacki or Iga Swiatek in the round of 16, with possible matches against Serena Williams and Simona Halep later in the week.
Pliskova will play Anna Kontaviet in the round of 16, with either Bianca Andreescu or Kiki Bertens awaiting in the quarter-finals.
