The Romanian was scheduled to play Tuesday in New Haven, but says in a statement her Achilles is sore from "two weeks of many matches and I need to recover in time for the U.S. Open."
Halep won the Rogers Cup in Montreal on Aug. 12 and advanced to the finals in Cincinnati, losing in three sets Sunday to Kiki Bertens at the Western & Southern Open.
The New Haven tournament also lost American CoCo Vandeweghe to injury. She withdrew Sunday with a sore right ankle while leading in the third set of her first-round match with Magdalena Rybarikova of Slovakia.
Fourth seeded Karolina Pliskova of the Czech Republic lost Monday to Ekatereina Makarova of Russia 6-1, 6-3.
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