Training, ✔️. Time to go! We have 21 #USTennis stars in action at the #USOpen on day one, including past Open champs @sloanestephens, @serenawilliams, and @venuswilliams. Tune-in to @espn networks starting at 11am ET to catch all the action. #tennis #venuswilliams #williams #venus #serena #sloanestephens #stephens