Andy Murray has announced his withdrawal from next week's Rogers Cup.
Croatian pair top Bryan Brothers to cut USA lead to 2-1.
Del Potro, Pella edge Italians Fognini, Lorenzi giving Argentina 2-1 lead.
In all-American clash, Young surges into first semifinal since 2015.
World No. 5 Simona Halep is the latest to announce her Rio withdrawal.
Two seeds fail to survive Hamburg quarterfinals, Renzo Olivo survived near facial trauma.
Milos Raonic becomes the latest top tennis player to withdraw from the Olympic Games in Rio.
