Williams rallied to defeat 60th-ranked Chanelle Scheepers 4-6, 6-2, 6-3. After Kevin Anderson beat Isner 7-6 (0), 7-6 (5), the match came down to mixed doubles. Williams and Isner beat the South Africans 6-3, 6-2 to clinch victory.
Williams took some time to settle in against an opponent who rallied well and hit the ball deep. Scheepers broke Williams at love to serve for the first set. But after that, the seven-time Grand Slam champion looked increasingly comfortable and began to play more aggressively.
Williams clinched the second set when Scheepers double-faulted. She then won the first four games of the third.
Williams was playing for the first time since winning at Luxembourg in mid-October.
"I was just trying to find some rhythm out there," she said. "She's obviously a really good player having maintained a top-40, top-50 ranking the last couple of years. She played a match yesterday, too, so had an extra advantage. It felt good out there.
"I'm not expecting to be perfect now, but I'm thinking that my game will get better every match. I think the doubles will help as well to hone everything. More than anything I was just so excited to be out there playing, and I'm ready to go for it."
In the late contest, Spain beat France 2-1 after Fernando Verdasco and Anabel Medina Garrigues teamed up with to win the mixed doubles against Jo-Wilfried Tsonga and Mathilde Johansson.
Tsonga put France ahead when he beat Verdasco 7-5, 6-3 in the singles, and Medina Garrigues levelled by overcoming Mathilde Johansson 6-3, 6-2.
Tsonga saved three set points before going up 1-0. He then broke three times in the second set and closed out the match on his fourth match point.
"It was not easy at the beginning," Tsonga said. "He played well. Then I wake up and played a lot better and it was a good match for me, to win the first match of the season. The first one is always tough. You have to be really focused. The best part of my game was that I moved pretty well today."
Medina Garrigues was rarely under pressure against Johansson, breaking to lead 4-3 and then taking the opening set when Johansson hit a double-fault on break point.
Three consecutive breaks of serve in the second set left the Spaniard leading 3-1, and she broke once more at love to claim victory.
On Monday, top-ranked Novak Djokovic and former No. 1 Ana Ivanovic represent Serbia against the Italian team of Andreas Seppi and Francesca Schiavone.
Andrea Petkovic, who retired with a right knee injury after winning the first set against Australia's Ashleigh Barty on Saturday, has withdrawn from the competition and will be replaced by Tatjana Malek for Germany's remaining contests against Italy and Serbia.
Petkovic, who ruptured her meniscus, will miss the Australian Open and is returning to Germany for treatment.