But team captain Kathy Rinaldi confirmed her participation after Venus won her singles match against Richel Hogenkamp to seal overall victory for the US.
Williams beat Arantxa Rus 6-1, 6-4, her 776th win in her 1,000th singles, 24 years after her first.
Perhaps wary of setting another ambitious target, Williams refused to be drawn on whether she had set her sights on the year's remaining Grand Slams - the French Open, Wimbledon and US Open.
"We saw her play an exhibition at the end of last year, but this is a great way for her to start the year and get a couple matches or a match behind her".
She added: "Right now I'm focused on this weekend and after that I'll figure out what it might be".
Rinaldi did not rule out the chance of Williams playing singles on Sunday, saying only that adjustments to the current lineup, if any, would only be made after Saturday's action. She and Lauren Davis are scheduled to face Lesley Kerkhove and Demi Schuurs in doubles.
Serena, 36, hasn't played an official match since winning the 2017 Australian Open.
The competition's most successful teams won their opening singles in World Group first-round home ties, the Americans against the Netherlands, and the Czechs against Switzerland.
Why most Wolves fans do not want West Brom to go down
To do this we must have personality to play with the ball. "We are facing a serious situation". "Now, it's different. In January, Chelsea signed Ross Barkley from Everton, Emerson Palmieri from Roma and Olivier Giroud from Arsenal .
Sunday's schedule now has Venus Williams facing Hogenkamp and Vandeweghe taking on Rus, but both captains still have time to make substitutions.
Before the match began, both teams were introduced to the sold-out crowd at the US Cellular Center in Asheville, NC. "We'll sit down and make that decision looking forward".
Venus Williams broke Rus five straight times to overcome a shaky serve.
In the first set, Hogenkamp won four straight games from 4-2 as Vandeweghe took advantage of the one break point she had, but looked too unsteady to hold. "There's a little bit of altitude here and I like to go big".
"Knowing I've got this handsome baby to go home to makes me feel like I don't have to play another match. I think it actually is a really flawless opportunity to try to come back". Finally, I just had to let go of that thought and just try to hit it hard and deep. "On paper I looked like I should win the match but it was a battle and I'm glad the U.S. won the battle".
Vandeweghe had a much more frustrating time.
Williams will be bidding to emulate Margaret Court, Evonne Goolagong and Kim Clijsters, who all won Grand Slam titles after having children, when she returns to the circuit. "It was better today than yesterday, when I played in the Fed Cup again after such a long time away and needed time to get used to it", she said.
Vandeweghe offset 12 double faults with 10 aces but struggled with 42 unforced errors. The raucous crowd heartily welcomed the USA team, starting with Captain Kathy Rinaldi, followed by Lauren Davis, Coco Vandeweghe, Venus Williams and finally Serena Williams.