Serena Williams loses to Raducanu; US Open next

U.S Open champion Emma Raducanu in the Western

The Cincinnati event was the second U.S. Open tuneup for Williams

Williams beat Nuria Parrizas-Diaz for her first match 

Raducanu, ranked No. 19 in the world

Williams did not speak to reporters after the match.

She briskly left the court after the match

Williams was sidelined for a year by a torn hamstring