The move comes six months after the Westmount, Que., native hired Sumyk after she had fired long-time coach Nick Saviano in November, 2014.
Bouchard has struggled this season and has seen her world ranking slip from No. 7 to No. 26.
An abdominal injury forced the 21-year-old to withdraw from the Citi Open last Friday for a second straight year.
Bouchard is set to compete in the Rogers Cup in Toronto, which starts Saturday. She earned a spot in the tournament after being Canada's top-ranked female player.
A trip to the Wimbledon final last year and semifinal berths at her first three 2014 Grand Slam events put Bouchard solidly on the tennis map, but she hasn't been able to live up to those expectations, losing five of her last seven events.
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