Andy Murray faces a countdown to fitness after pulling out on Tuesday from the Cincinnati Masters with an abdominal injury.
The 2008 and 2011 champion at the flatland Midwest venue had been carrying the problem since last week in Toronto.
Murray gave a third-round walkover to Jannik Sinner due to his physical problem.
The 36-year-old Scot now has 13 days until the August 28 start of the US Open and will be fighting tro heal in time for the Grand Slam start.
Murray was replaced in the draw by German Daniel Altmaier, who starts against Aussie Alexei Popyrin.
