US Open champ Sloane Stephens wins Cincinnati opener

Defending U.S. Open champion Sloane Stephens overcame a thumb injury to cruise into the third round of the Western & Southern Open with a 6-3, 6-2 win over qualifier Tatjana Maria on Wednesday.

Third-seeded Alexander Zverev was less fortunate, slipping to 0-4 in Cincinnati with a 5-7, 6-4, 7-5 second-round loss to Robin Haase.

A trainer applied a bandage to the third-seeded Stephens' right thumb between the third and fourth games of the second set of this U.S. Open tuneup. The match was Stephens' first since losing on Sunday to Simona Halep in the finals at Montreal.

Alexander Zverev, of Germany, reacts during a match against Robin Haase, of the Netherlands, in the second round at the Western & Southern Open tennis tournament, Wednesday, Aug. 15, 2018, in Mason, Ohio. (AP PhotoJohn Minchillo)

Sloane's semifinal appearance last season is her best Cincinnati finish in six previous appearances.

Ninth-seeded Karolina Pliskova was upset by Aryna Sabalenka, 2-6, 6-3, 7-5 in a second-round match. Fifteenth-seeded Elise Mertens rallied to squeeze past qualifier Rebecca Peterson, 3-6, 6-2, 7-6 (1) into the third round.

Among the men, seventh-seeded Marin Cilic shook off dropping a first-set tiebreaker to get past qualifier Marius Copil, 6-7 (4), 6-4, 6-4 and reach the third round.

Tatjana Maria, of Germany, reacts during a match against Sloane Stephens in the second round at the Western & Southern Open tennis tournament, Wednesday, Aug. 15, 2018, in Mason, Ohio. (AP PhotoJohn Minchillo)

Sloane Stephens reacts during a match against Tatjana Maria, of Germany, in the second round at the Western & Southern Open tennis tournament, Wednesday, Aug. 15, 2018, in Mason, Ohio. (AP PhotoJohn Minchillo)

Karolina Pliskova, of the Czech Republic, returns to Arena Sabalenka, of Bulgaria, in the second round at the Western & Southern Open tennis tournament, Wednesday, Aug. 15, 2018, in Mason, Ohio. (AP PhotoJohn Minchillo)

Arena Sabalenka, of Bulgaria, reacts during a match against Karolina Pliskova, of the Czech Republic, in the second round at the Western & Southern Open tennis tournament, Wednesday, Aug. 15, 2018, in Mason, Ohio. (AP PhotoJohn Minchillo)