Defending champions Grigor Dimitrov and Garbine Muguruza will take the court on Wednesday, headlining a packed schedule also featuring world No. 1 Simona Halep and No. 3 Juan Martin del Potro. Keep an eye on these matchups:
 Grigor Dimitrov vs Mischa Zverev
Grigor Dimitrov’s dynamic title run last year in Cincinnati catapulted him to a career-high rank of No. 3 in the world at the end of last season. The 27-year-old Bulgarian has seen some inconsistent results in 2018, admitting last week in Toronto that he still hasn’t quite found his rhythm this summer. His first opponent—Germany’s Mischa Zverev—likely won’t make a rhythm renaissance any easier. One of the few players on tour with a game centered around net play, Zverev is coming off a strong grass season that saw him capture his first career title in Eastbourne. He’ll be nice and warm on Wednesday after notching a comprehensive win over Damir Dzumhur in the first round.
 Garbine Muguruza vs Lesia Tsurenko
Spain's Garbine Muguruza will open her title defense--and her hard-court summer swing--against Ukraine's Lesia Tsurenko. The 24-year-old is looking to collect her second title of the year and her third straight win over Tsurenko, who she also defeated this year at Roland Garros.
 Juan Martin del Potro vs Hyeon Chung
Three years ago, Juan Martin del Potro sat at home recovering from his third surgery on his left wrist. On Monday the 29-year-old rose to a new career-high rank of No. 3 in the world. A two-time semifinalist here in Cincinnati, del Potro will meet South Korea’s Hyeon Chung for the first time on Wednesday. Chung, 22, announced his arrival on tour with a run to the semifinals at the 2018 Australian Open, but Chung has had injuries of his own, sitting out most of May, June and July with ankle pain. Who will be the last man standing?
 Caroline Wozniacki vs Kiki Bertens
Caroline Wozniacki kicked off her 2018 season in style, capturing her first Grand Slam at the Australian Open and rising to No. 1 in the world soon after. Still looking for her first hard court win of the summer, the 2014 semifinalist and No. 2 seed will face a stern test in her opening match in the form of red-hot Kiki Bertens. The 26-year-old from the Netherlands topped Wozniacki this spring en route to the Madrid final, also capturing a title in Charleston and reaching the quarterfinals at Wimbledon, where she upset Venus Williams and Karolina Pliskova. Bertens’ 6-2, 6-0 drubbing of CoCo Vandeweghe in the first round should put Wozniacki on notice.
 Simona Halep vs Ajla Tomljanovic
Fresh off her blockbuster win over Sloane Stephens in the Montreal final just days ago, Simona Halep is playing for another shot at the Rookwood Cup. The two-time Cincinnati finalist opens play against 25-year-old qualifier Ajla Tomljanovic. After sitting out the 2016 season with a shoulder injury, Tomljanovic is closing in on her career high rank of No. 47 in the world. Her first-round win against Irena Camelia Begu was her first in Cincinnati—the world No. 1 stands in the way of her second.