Serena Williams’ shock loss to 18-year-old Belinda Bencic in the Rogers Cup semifinal has shaken up women’s tennis just in time for the 2015 Western & Southern Open. When she won in Cincinnati last year, the American became one of 11 different women to have won the event since the WTA tour made its first visit to the city in 2004, and this year could see a 12th different champion.

Before the world No. 1’s unexpected loss, that seemed unlikely, but it certainly feels like Serena’s 2015 momentum has been somewhat slowed. Nevertheless, the defeat merely serves to demote her status from “near-certain winner” to “heavy favorite.”

Toronto champion Bencic will be among the chief challengers for the Cincinnati title after producing the best tennis of her life to beat an extremely impressive sequence of opponents – Eugenie Bouchard, Caroline Wozniacki, Sabine Lisicki, Williams, and Simona Halep – and win her first WTA Premier Five title.

The Swiss youngster’s superb achievement has taken her to No. 12 in the world rankings, but she will nevertheless need to be playing her best from the outset in Cincinnati, since she has been drawn against in-form Angelique Kerber in the first round. And even if she wins, her quarter of the draw is undoubtedly the toughest. Along with Bencic herself, it contains three of the game’s most talented young players – Caroline Garcia, Bouchard, and Elina Svitolina – as well as Petra Kvitova, Lucie Safarova, and Kerber. And to make matters worse, whoever emerges victorious from this section is likely to face Serena in the semifinal.

The American’s status as top seed has earned her a very favorable draw in Cincinnati, and she should reach the quarterfinal without dropping a set. Once there, Serena is almost certain to face a dangerous opponent, with her sister Venus, Carla Suarez Navarro, Sloane Stephens, and Ana Ivanovic fighting for the right to face her. If, as expected, she reaches the semifinal, it will be very interesting to discover, and almost impossible to predict, who her opponent is.

World No. 2 Simona Halep’s excellent return to form would have made her one of the favorites for the Western & Southern Open if she had not retired when 0-3 down in the deciding set of the Toronto final, but her knee injury casts doubt on both her ability to compete in Cincinnati.

Agnieszka Radwanska is a strong bet to do well again, particularly given her favorable draw. She faces a qualifier first, and then probably Alize Cornet next. She is due to face Maria Sharapova in the last 16, but considering the Russian has been nursing an injury and has not played since Wimbledon, there is no guarantee she will make it far.

Caroline Wozniacki is in a similar situation to Sharapova, although she claims her calf is feeling fine now, and she could hardly have a tougher draw. She could face Victoria Azarenka in round two, Garbine Muguruza in round three, Simona Halep (if fit) in the quarterfinal, and Sharapova or Radwanska in the semifinal. It would take some of the best tennis of her career to overcome such high-quality opponents.

If neither Halep nor Wozniacki are 100% fit, Azarenka and Muguruza should be motivated to take full advantage of their excellent opportunity to reach at least the semifinals. Andrea Petkovic and Anastasia Pavlyuchenkova could also pose a threat in that section of the draw, but it is still a draw that invites the Belarussian and the Spaniard to go far. This pair of big-hitters will also fancy their chances against Radwanska, if she makes it that far, in the semifinal. If Sharapova is their opponent, however, they will be far less confident of victory, considering their poor records against her.

Recent events have definitely made this year’s Western & Southern Open far more unpredictable than anyone would have expected when the US Open Series began. However, it could still be argued that the most likely semifinals, based on ability and recent form, are both contests that happened in Toronto – Serena vs. Bencic and Halep vs. Radwanska.

If this is how the last four line up, can Bencic beat Serena again? Or will the world No. 1 produce her best tennis and re-assert her authority in women’s tennis? And will Halep and Radwanska both play well simultaneously if they play each other again? They could not during a three-set rollercoaster in the Rogers Cup quarterfinals.

Arguably, in terms of the US Open itself, it does not matter much how well these four players perform because they have already found their rhythm ahead of the year’s final Grand Slam. The most interesting storylines to follow are the players who are struggling for fitness and form – the likes of Wozniacki, Sharapova, Muguruza, and Ivanovic, since their weeks in Cincinnati could have a huge bearing on how well they play at Flushing Meadows.