Ash Barty's surprise defeat to Sofia Kenin in the second round of the Rogers Cup in Toronto means the Aussie could lose her number one WTA ranking by time the tournament is done.
After coming from a break down to claim the opening set against 20-year-old Kenin, Barty was eventually ousted from the tournament in three sets.
In claiming a 6-7(5) 6-3 6-4 win, Kenin became the youngest player in eight years to beat a current world number one.
And with seven of the top eight players in the world in the draw, the result means Barty could relinquish top spot in the rankings.
MORE: Ash Barty suffers surprise Rogers Cup exit
With Barty's exit, all eyes will be on Naomi Osaka - the woman she wrested top spot from in June - and Karolina Pliskova, who could rise to number one for the second time in her career.
It all rests on how well Osaka plays over the next couple of days.
The reigning US Open and Australian Open champion will open her Rogers Cup campaign against German Tatjana Maria on Thursday morning (Australian time).
If Osaka wins her opening match, Barty will lose top spot.
It will then come down to who does better out of Osaka and Pliskova, although Osaka has the advantage.
Pliskova must advance to the final four to have any hope of reclaiming the number one ranking. If Osaka reaches the quarter-finals, Pliskova must make the final to take top spot.
If Osaka and Pliskova both make the final, the winner will be the new world number one.
But, there is a small chance Barty can maintain her position atop the rankings.
If Osaka loses her first round match, and Pliskova fails to reach the semis, the Aussie will remain as world number one.
Is there a chance it could happen?
Of course there is. Osaka hasn't been in great form lately, having been knocked out of Wimbledon in the first round, eliminated in her second match at Birmingham and ousted in the third round at the French Open.
However, the Rogers Cup is back on her favoured hardcourt surface, where she has won both of her Grand Slam titles.
Osaka has a win and a loss against Maria, both on hard court and both in 2016.
Pliskova had a first round bye and will play Alison Riske, who beat Barty at Wimbledon, in the second round.
She then has potential matches against Carla Suarez Navarro, Anna Kontaveit, Daria Kasatkina or Kiki Bertens.