Angelique Kerber eyeing Serena Williams' world number one status

Top seed Williams, 34, revealed she had opted against accepting her wild-card place at this tournament due to a shoulder injury and if Kerber goes on to win in Cincinnati, she will take over at the top of the WTA rankings table. Williams has been ranked world number one for 183 consecutive weeks.

Kerber's second-round opponent will be either Kateryna Bondarenko or Kristina Mladenovic, while Jelena Ostapenko enjoyed a memorable win on Monday.

The 19-year-old former junior Wimbledon champion produced a stunning comeback to defeat Anna Karolina Schmiedlova 1-6 7-6 (8/6) 7-6 (7/5) and she will next meet Karolina Pliskova of the Czech Republic.

Lesia Tsurenko downed qualifier Shuai Zhang 6-3 6-3 as American hopeful Coco Vandeweghe defeated former French Open finalist Sara Errani 4-6 6-3 6-4.

Elsewhere, Daria Gavrilova ousted Caroline Garcia on the back of a 7-5 6-3 triumph and Donna Vekic surprised Ana Ivanovic to win 6-4 6-2.

Christina McHale rounded off the day's play with a regulation 6-2 6-2 success against qualifier Saisai Zheng.