World number one Andy Murray will miss next week's ATP Masters Series event in Cincinnati with a nagging hip injury that also forced him out of this week's Montreal event.
The move brings into question Murray's status for the US Open, which starts August 28 in New York, and even if he is able to play, it remains to be seen how well he can perform after missing the two major hardcourt warm-up events for the Flushing Meadows fortnight.
"Unfortunately I won't be playing in Cincinnati as I continue my recovery," Murray said Wednesday. "I always enjoy playing there and I look forward to returning next year.
"I'm continuing to work hard on the court with the aim of being in New York."
The 30-year-old Scotsman, who has topped the rankings since last November, could be overtaken this week by French Open champion Rafael Nadal if the Spaniard reaches this week's Montreal semi-finals.
Nadal will be the number one seed next week in Cincinnati no matter how he fares in Canada this week. It will be the first time since 2010 that Nadal, who won at Cincinnati in 2010, has been top seed there.
Wimbledon and Australian Open winner Roger Federer will be the second seed. He has won seven Cincinnati titles, including crowns in each of his past two appearances there in 2014 and 2015.
This marks the first year since 2009 that Nadal and Federer are the two top seeds in Cincinnati.