Perth - World number eight Jack Sock's Australian Open preparation suffered a setback when he was injured at the Hopman Cup in Perth on Tuesday, resulting in a surprise call-up for Australian veteran Pat Cash as a substitute.
On a chaotic day, Sock retired early in the second set of his men's singles clash with Yuichi Sugita, having injured his hip during his loss in the first set tie-breaker at the mixed teams tournament.
Earlier, Japan's Naomi Osaka had withdrawn from her singles match with CoCo Vandeweghe due to illness.
The injury and illnesses meant the United States were awarded the match 2-1 via two walkover wins in the women's singles and mixed doubles.
And it also meant an unscheduled return to action by the 1987 Wimbledon champion Cash, who played in the first Hopman Cup back in 1989.
He now coaches Vandeweghe and teamed up with her for an exhibition mixed doubles to entertain the crowd against young Australian Maddison Inglis and Sugita.
But the fitness of Sock for the remainder of the tournament, and more importantly the upcoming Australian Open, remained the biggest concern after he limped off the court.
Both Sock and Vandeweghe, who beat teenager Inglis in their exhibition singles in the absence of Osaka, slipped and fell several times during their matches.
Sock, who had squandered an early service break in the first set, went to ground early in the tiebreak, reaching immediately for his right hip.
He had treatment after dropping the tiebreak 7-1 and struggled through two more games before retiring.
Sock joins a long list of leading men's players with injury concerns heading into the year's first Grand Slam, including number one Rafael Nadal, Novak Djokovic, Andy Murray, Stan Wawrinka and Milos Raonic.
With Osaka ruled out, local 19-year-old Inglis, ranked 771st, was given the daunting task of facing world number 10 Vandeweghe.
"She played really well and was a tough opponent, it's not easy to come out at the last minute," Vandeweghe said after winning 7-5, 6-2.
"She made it difficult and I didn't make it easy for myself either."
The Americans beat Russia 2-1 on Sunday and are seeking to reach the final for the second successive year, having been beaten by the French team of Kristina Mladenovic and Richard Gasquet in last year's decider.
They next are due to face the Swiss team of Roger Federer and Belinda Bencic on Thursday.
Scores from the Hopman Cup mixed teams tournament in Perth on Tuesday:
United States bt Japan 2-1
Women's singles: CoCo Vandeweghe (USA) bt Naomi Osaka (JPN) 6-0, 6-0 (W/O)
Men's singles: Yuichi Sugita (JPN) bt Jack Sock (USA) 7-6 (7/1), 1-1 (ret)
Mixed doubles: Vandweghe/Sock (USA) bt Osaka/Sugita (JPN) 4-0, 4-0 (W/O)