​It is No.1 vs. No.2 as Serena Williams and Simona Halep will battle it out in the Western & Southern Open final in Cincinnati on Sunday.

Defending champion Serena Williams could become the first player to win the Cincinnati title twice, if she manages to hold off Simona Halep in the final on Sunday. No player since the tournament’s inaugural edition in 2004 has wielded to achieve the feat.

Williams dropped just the single set last year on her way to brushing aside Ana Ivanovic 6-4, 6-1 in the final. But she noticeably hasn’t been her dominant self throughout the US Open Series this season, punctuated by her shock loss against WTA Rising Star Belinda Bencic in Toronto last week. Poor serving has also landed Williams in trouble in Cincinnati this week, but the World No. 1 has nevertheless managed to reach the final for the third year in a row, having only dropped the single set (in a rematch against Ivanovic in the quarterfinals).

In her semi-final meeting with Elina Svitolina, it was the World No. 20 who came out with all of the impulse and momentum. But the defending champion retrieved her calmness to win the next eleven out of 15 games to secure her place in the final.

But there is still one more vault that Williams needs to overcome: Simona Halep. After a brief slouch in form, which saw her left with no wins at Wimbledon, Halep has rebounded back in recent weeks. She defeated consecutive top 25 opponents Jelena Jankovic, Angelique Kerber, Agnieszka Radwanska and Sara Errani on her way to the Toronto final last week. She retired due to illness in the final against Belinda Bencic, but Halep has sprung back, despite the quick turnaround, to reach her second WTA final in as many weeks.

In doing so, Halep has also managed to recover the No. 2 ranking from Maria Sharapova, which could therefore have major significances for both herself and Sharapova’s US Open campaigns. With the Grand Slam seedings to be announced next week, Halep can effectively avoid a clash with Williams until potentially in the final, while Sharapova is not warranted the same safety. If the former US Open champion is drawn in Williams’ top half of the draw, she could be on track to face her nemesis earlier than preferred. Williams holds the dominant 17-0 winning streak against Sharapova.

Halep defeated Kristina Mladenovic 7-5, 5-7, 6-4, Andrea Petkovic 4-6, 6-4, 6-2, Anastasia Pavlyuchenkova 7-6 (3), 6-2 and Jelena Jankovic 6-1, 6-2 to reach the final for the first time. But she only ever defeated Williams once (at the WTA Finals in Singapore last year) in six meetings, and it was the World No. 1 who was successful when they met earlier this year in Miami (she did get a walkover from Williams at Indian Wells).

But can Williams be able to turn around her disastrous serving percentage to win her fifth title of the season ahead of the US Open?