Australia's Barty, 23, won the battle of former number one players in just over 90 minutes.
The reigning Roland Garros champion beat five-time Grand Slam winner Sharapova, who was Cincinnati champion in 2011, for the second time this season after a win at the Australian Open.
It was Barty's 40th victory of 2019 and she goes on to face either Poland's Iga Swiatek or Anett Kontaveit of Estonia.
Results on Wednesday in the WTA/ATP Cincinnati Masters (x-denotes seeding):
Richard Gasquet (FRA) bt Federico Delbonis (ARG) 7-5, 7-6 (7/1)
Yoshihito Nishioka (JPN) bt Kei Nishikori (JPN x6) 7-6 (7/2), 6-4
Alex De Minaur (AUS) bt Reilly Opelka (USA) 7-6 (7/3), 6-4
Aryna Sabalenka (BLR x9) bt Zheng Saisai (CHN) 6-4, 6-3
Karolina Pliskova (CZE x3) bt Wang Yafan (CHN) 6-1, 6-3
Rebecca Peterson (SWE) bt Veronika Kudermetova Read More – Source