Cincinnati - Canadian teenager Bianca Andreescu made history with a Toronto WTA title on Sunday and then withdrew from next week's big date at the Cincinnati Masters.
The 19-year-old who claimed the trophy when Serena Williams was unable to play more than four games before retiring with back spasms said "Thanks, but no thanks' to any extra exertion in the US Open run-up."
The first Canadian in half a century to win the home trophy said she did not want to take any risks after missing more than two months - and Wimbledon - with shoulder problems before returning to action this week.
"I've never been there, so I'm really disappointed to withdraw from the tournament," said Andreescu, who was leading Williams 3-1 when the 23-time Grand Slam champion called it a day.
"I really have to listen to my body right now, this last week has not been easy on it," she added after beating three top 10 players in her hometown win that thrilled a nation.
Williams, 37, said she would make the trip to Cincinnati before deciding whether she would play.
"The most frustrating part is that I've had these awful spasms a lot in my career, actually," she said.
"They're incredibly painful, but it goes away after, like, 24, 36, maybe 48 hours, like clockwork.
"I do different treatments. I take a day off. But obviously, I didn't have 24 hours or plus to take off (in Toronto).
"The first phase is painful, to a point where I usually don't get out of bed. I'll have to wait and see."
Williams won Cincinnati in 2014 and 2015 but went out in the second round to Petra Kvitova last year.