The USA's chances of claiming a seventh Hopman Cup title took a hit on Tuesday when world No.8 Jack Sock was forced to retire during his clash with Japan's Yuichi Sugita.
Sock injured his hip when he took a fall during the first set of his men's singles clash.
After taking a 4-2 lead, the Japanese world No.40 claimed the set in a tiebreaker.
The American took treatment during the break, but then called it a day when trailing 7-6 (7-1) 1-1.
Earlier, CoCo Vandeweghe was officially handed a 6-0 6-0 victory in the women's singles after Japan's Naomi Osaka pulled out of the day's play due to illness.
And because Osaka forfeited first, the US team had been handed a 6-0 6-0 doubles victory before Sock injured himself.
Team USA will face Switzerland in a sell-out Thursday night tie with a place in Saturday's final likely to be on the line.
If Sock can't take his place, the Swiss pair of Roger Federer and Belinda Bencic will likely claim their spot in the playoff as the highest-ranked team in Group B.
Vandeweghe teamed up with her coach and 1987 Wimbledon champion Pat Cash in a dead fast4 mixed doubles rubber against Sugita and West Australian teenager Maddison Inglis.
Earlier, despite the withdrawal of her opponent, Vandeweghe still received a solid workout on the court.
The world No.10 was put through her paces by 19-year-old Inglis, who has been used as a hitting partner during the tournament this summer and last.
Inglis, ranked 771 in the world, broke the serve of her more experienced opponent once in each set and with a bit of luck could have won the opening set.
But Vandeweghe delivered on important points to claim a 7-5 6-2 victory.
Vandeweghe admitted that she has some work to do on the mental side of her game as the Australian Open gets nearer.
"I have been practising with Maddi all week, so she is comfortable with me and my game; I would think," she said.
"She is quite a different opponent to Naomi. But I'm just trying to do the best I can against any opponent who comes my way.
"It's more, for me, more a mental side, focusing on actual play. I got a bit sloppy out there, especially today.
"I think that's just the start of the season."
Vandeweghe sent down four aces and hit 18 winners in that opening set, but still took 55 minutes to take the lead.
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