The 36-year-old last played competitively at the 2017 Australian Open, having given birth to her first child in September.
If the USA wins its first singles match Sunday to clinch the victory in this best-of-5 event, Sunday's second singles match would be scrapped but the doubles match would still go on. The U.S. advanced to a World Cup semifinal April 21-22 at France, which defeated Belgium 3-2 this weekend.
"It's never easy and it's always a challenge", Williams said after beating her 124th-ranked opponent.
Former world number one Serena Williams, who is looking to return to the top of her sport after taking a break due to her pregnancy, has not played a competitive match since she won the Australian Open title past year for her Open-era record 23rd grand slam.
But that is no indication of Williams' inevitable return to singles action after the 24-time Grand Slam champion confirmed she was undecided over her participation in this year's three remaining majors.
Williams had told Vogue magazine that she developed several small blood clots in her lungs after her daughter was born.
Hogenkamp won four consecutive games to take the opener but Vandeweghe levelled the match with an ace on her fourth set point, triggering a massive fist pump and show of emotion that few expected from what initially appeared to be a mismatch.
Williams and Vandeweghe won their singles matches Saturday to give the US a 2-0 lead over the Netherlands.
Sunday's scheduled singles match between CoCo Vandeweghe and Arantxa Rus was scrapped because the outcome of this event already has been decided.
And Captain Rinaldi stuck to the original schedule, sending No. 8 Venus Williams to try and clinch the tie against Richel Hogenkamp. Slated to play doubles with Lauren Davis, chatter was rampant as to whether the younger Williams would slide in for singles play.
"We'll decide tonight", USA captain Kathy Rinaldi said after Saturday's matches.
Aryna Sabalenka put Belarus 1-0 with a 4-6, 6-1, 6-2 win over Tatjana Maria, but Antonia Lottner levelled the tie with a 7-5, 6-4 win over Aliaksandra Sasnovich.
Right now, I don't know.
Mertens' victory was her fifth in a row in Fed Cup singles. Williams finally saved a break point in the sixth game and held serve for the first time in the second set.
"You're under pressure a lot", Hogenkamp said. One of those matches proved much closer than the other.
She is now one title short of matching Court's all-time record for Slam singles titles of 24 - a mark she is determined to better. That's a different feeling for me.
The Czech Republic advanced to the Fed Cup semifinals after double Wimbledon champion Petra Kvitova beat Switzerland's Belinda Bencic 6-2, 6-4 Sunday.