Injury forces Halep out of Bad Homburg ahead of Wimbledon
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"I am sorry I had to withdraw today from the semi-final match," Halep wrote on Twitter.
"But unfortunately I woke up this morning with a blocked neck and this is not allowing me to perform to the best of my abilities," she added, without saying whether she can play next week.
With Romania's Halep sidelined, her Canadian opponent Bianca Andreescu progressed to the Bad Homburg final on Saturday, the WTA said in a brief statement.
She will face France's Caroline Garcia, who came out on top of a nerve-wracking third set to seal a 7-6 (11/9), 3-6, 7-5 victory over compatriot Alize Cornet in Friday's semi-final.
After losing the first on a tie-breaker, Cornet took the second and had a match point at 5-4 up in the third with Garcia serving.
However, the 28-year-old held her service, broke Cornet in the next game and then served out to continue her bid for a first title on the WTA tour.