Together, they gave the USA a 2-0 lead over Latvia after the first day of action Friday night in the Fed Cup qualifier at Angel of the Winds Arena.
She hadn't played singles in the women's nations event since 2015, but with the Tokyo Olympics on the horizon the United States veteran was all-in on tennis as "a team sport now".
The 23-time Grand Slam champion was more dominant in the second set, allowing Sevastova to hold just one service game, but lost the decider in another tough tie-break. Williams served for the match up 6-5 in the second set, but Ostapenko, 16 years younger, broke Williams' serve to set up another tiebreaker. "I was obviously a little nervous coming in after Australia, and I felt exhausted, but I tried to get that out of my mind and do what I do best".
"Obviously I'd love to give her an experience at Fed Cup, especially her first one, but our goal is we're one team, one goal, and that's to win", Rinaldi said. The key point of the tiebreaker was Williams' forehand victor up the line for a 5-2 lead that was followed by one of her trademark fist-pump and scream. "So, we should take confidence from this match and I'm really looking forward to April". She struggled with Ostapenko's power before adjusting to dominate the second set, but she couldn't hold serve in the final set.
Sevastova used her powerful serve to deliver aces at key moments, and a precision forehand ended Williams' ideal streak at 14 wins after a 2hr 25min marathon.
Williams' loss to Sevastova comes just weeks after she was shocked by China's number 27, Wang Qiang, at the Australian Open 32.
Doubles deciding the Washington meeting was a surprise after both Kenin and Williams won on opening night.
But Sevastova ended Williams' unbeaten run.
Williams had chances to clinch a victory for the USA and wrap up its spot in the finals.
Sorribes dropped just three games in total as she looked at ease on court and thrashed Osaka 6-0 6-3.
The pair exchanged breaks in the first three games of the third, leaving Ostapenko with a 2-1 lead. Kenin made the a lot 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. She almost gave the advantage back to Kenin, but Ostapenko saved a pair of break points to take a 3-1 lead on a swinging forehand volley.
But Williams won five straight points to take a 5-2 lead in the decider, and Ostapenko double-faulted to give her a match point that Williams seized.
The big-hitting, former French Open champion gave Williams all she could handle in the second singles match on Friday, losing because of errors in both tiebreakers.
This approach may have contributed to their 49 casual mistakes.
"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.
"She was playing some really good tennis and I knew I just needed to hang in there", Kenin said.