Sofia Kenin and Bethanie Mattek-Sands defeated Jelena Ostapenko and Anastasija Sevastova 6-4, 6-0 in a deciding doubles as the USA survived a Saturday scare to beat unheralded Latvia 3-2 and reach the Fed Cup finals.
New Australian Open champion Kenin and Mattek-Sands ensured the USA reached the finals in Budapest from April 14-19 after both Serena Williams and Kenin had been stunned in the singles as Latvia levelled the qualifier 2-2 in Seattle.
Sevastova had earlier inflicted a shock first Fed Cup singles loss on Williams 7-6 (7/5), 3-6, 7-6 (7/4).
"We were playing in the USA so obviously the crowd was going to be for Serena. I just tried to give it my best," Sevastova said of her win at the Angel of the Wings arena.
Williams, the 23-time Grand Slam champion, bounced back from losing the first set by cruising through the second set in 33 minutes, but she was outplayed on the big points in the second tiebreak.
Sevastova used her powerful serve to deliver aces at key moments, and a precision forehand ended Williams' perfect streak at 14 wins after a 2hr 25min marathon.
Williams looked fatigued in her first Fed Cup singles match in four years. She had played doubles with sister Venus in her last tournament appearance in 2018.
Williams' loss to Sevastova comes just weeks after she was shocked by China's world number 27 Wang Qiang in the last 32 of the Australian Open.
- Kenin beaten -
The former French Open champion Ostapenko had kept Latvia alive with a 6-3, 2-6, 6-2 upset against Kenin in just under two hours after the Americans went into the second day with a 2-0 lead.
"Honestly it's very special to play for my country and I was just trying my best," said Ostapenko, who raced to a 3-0 lead against Kenin and broke her again in the eighth game on the way to winning the first set.
"I was fighting until the last point," she said. "I knew it was going to be a very tough match and the key thing for me was just to play aggressive."
That approach may have contributed to her 49 unforced errors. Kenin made the most of them in the second set, fending off a break point in the first game then pulling away to take a 4-0 lead on the way to forcing a third set.
In a third set that featured six breaks of serve, Ostapenko held for 3-1 and, finally, held in the last game, sealing it on her second match point.
Double delight: Sofia Kenin (left) and Bethanie Mattek-Sands celebrate taking the USA to the Fed Cup finals
Shock defeat: Serena Williams reacts as she loses for the first time in Fed Cup singles action to Latvia's world number 41 Anastasija Sevastova