What a turnaround it has been at the WTA Finals in Singapore. Going into the last round of round-robin matches, Simona Halep and Angelique Kerber were the women most likely to join Maria Sharapova and Garbine Muguruza in the semifinals. However, an extraordinary set of results meant Agnieszka Radwanska and Petra Kvitova qualified instead.

The only way Radwanska could qualify from the Red Group was if she defeated Halep in straight sets and Sharapova also defeated Flavia Pennetta in straight sets. That is exactly what happened, and the Pole is no doubt very grateful to the Russian. In the White Group, Kvitova crucially grabbed a set in her loss to Muguruza, and then watched on delightedly as her friend and compatriot Lucie Safarova defeated Kerber in straight sets to send her through.

Muguruza and Sharapova are in such sparkling form that reaching the semifinals likely will be the limit of Radwanska's and Kvitova's achievements in Singapore, but they must still believe that anything can happen. The events of the last two days are proof of that.

If Radwanska does defeat Muguruza, she will end a sequence of four losses in a row against the Spaniard. To win, she will have to assess how Muguruza is playing and adjust her tactics accordingly. If the 2015 Wimbledon finalist is missing a lot of shots and seems to be suffering the effects of twisting her ankle against Kvitova on Friday, the Pole’s typical style of making her opponent play countless extra shots until she makes an error will probably be the right way to go. However, if Muguruza is hitting the ball well and playing somewhere near her best, Radwanska will have to be aggressive and proactive, or she will be blasted off the court by her opponent’s power.

While Sharapova’s form makes her the favorite for her clash with Kvitova, it is definitely the harder of the Saturday matches to call. Both players have similar styles that rely on pinning their opponents into awkward locations on the court and then firing winners into the resulting spaces. Both are tall and not the most mobile, despite considerable improvements over the years, particularly from Sharapova. Thus, the result may depend on who is more consistent when it comes to hitting winners. If both play well and the match goes to the wire, Sharapova’s superior mental strength should see her through.

Sharapova and Muguruza have been the stand-out players so far this week, so an encounter between them would be a fitting finale to an excellent week of tennis in Singapore. But Kvitova and Radwanska are both unpredictable opponents, and either could spring a surprise in the semifinals.