Perth - Spain's Hopman Cup title defence is all but over after they were beaten 3-0 by the United States on Monday.
The Spanish pairing of Anabel Medina Garrigues and Daniel Munoz-De La Nava were also beaten by the Czech Republic on Sunday and while they still have one group tie left, they cannot reach Saturday's final of the mixed teams tournament.
Medina Garrigues was thrashed 6-3, 6-1 by world number 12 Sloane Stephens in the women's singles.
The towering John Isner then cruised to a 6-3, 6-4 victory against the outclassed De La Nava, the world number 194 who was only called up for the tournament at the last moment to replace the injured Tommy Robredo.
In the mixed doubles, the Americans completed a clean sweep when they came from a set down to win 4-6, 6-0, 10-4 in a match tiebreak.
It was the first tie of the tournament for the United States, who are aiming for a record seventh title in Perth.
Stephens, who reached the semi-finals of the Australian Open in January, was far too good for the struggling Medina Garrigues, ranked 100th in the world.
Although the Spaniard was moving better than in Spain's opening tie when she battled knee soreness, she also appeared to be discomforted by the Perth heat and served several double faults late in the match.
The emerging Stephens had far too many shots for the veteran, hitting a number of impressive winners off both wings and wrapping up the match with a commanding overhead.
Medina Garrigues won all but one of her four singles matches as she teamed with Fernando Verdasco to win the title earlier this year, but has won just five games in her two outings this week.
Stephens, 20, was satisfied with her first outing of the new season.
"I am really excited to get my first win of the year here, I played really solid," she said.
"It was a little hot so I tried to keep my composure but I'm really happy with the way I played and happy to be back here in Australia.
"I thought it was a good way to start the year off."
The big-serving Isner, ranked 14th in the world, took just over an hour to dispose of Munoz-De La Nava.