Halep was set to face Veronika Kudermetova in the second round. Kudermetova received a walkover into the Round of 16 and will face either Paula Badosa or Ajla Tomljanovic.
Dutch challenger Kiki Bertens profited from a third-set collapse from Simona Halep to score the biggest win of her career Sunday, beating the world number one 2-6, 7-6 (8/6), 6-2 in the Cincinnati final.
Halep, who won the Montreal trophy a week ago and came into the final on a nine-match winning streak, ran out of reserves and was unable to stop the mounting Bertens momentum.
“I’ve lost three finals here, but maybe I will able to win one at this tournament because I love to play here,” said Halep, who fell to Serena Williams in the 2015 final of the hardcourt US Open tuneup and to Garbine Muguruza in the title match last year.
The upset took just over two hours and marked the 10th time this season that 17th-ranked Bertens has defeated a top 10 opponent.
“Hopefully we can play many more matches like this in the future,” said Bertens, who claimed a sixth career WTA title.
Bertens saved a match point in the second-set tiebreaker to stay alive.
Halep had looked like ending a quick afternoon with a victory after breaking Bertens twice to sweep the first set.
But the 26-year-old Dutchwoman, who had considered retiring last season due to poor results, began to turn the tide in the second as she broke for a 3-1 lead.
Halep got that break back in the eighth game to make it 4-all, and took tighter control in the tiebreaker.
But Bertens won four games in the decider to take a 4-2 lead and quickly seized the set seconds after saving the match point.
Halep seriously faded in the final set, going down two breaks to trail 1-4.
Bertens kept up the pressure and advanced with a winner on her first match point, posting her second career win over Hale six years after beating her for the first time.
The depleted Halep is still entered as top seed in the upcoming WTA tournament in New Haven, which ends just two days prior to the start of the US Open in New York.