The reward is a match-up with older sister Venus, after the eighth seed saw off Sorana Cirstea 6-3 6-4.
It will be the 29th time the Americans have met each other on the WTA Tour, as the younger of the sisters plays in her first competitive tournament since the birth of her daughter last September. "Obviously I have to play better than her and see how the match goes", Venus told the media after her victory.
Serena, who lifted her 23rd Grand Slam title in Melbourne past year, was then already pregnant with daughter Alexis Olympia, who was born in September. "You know, we can always stay in the tournament longer if the both of us are in the tournament", Serena said.
The late decision sparked conjecture that her withdrawal had been a family arrangement, a conspiracy theory given further momentum when the Russian player Elena Dementieva said after her quarterfinal loss to Venus Williams that she believed the sisters' father and coach, Richard Williams, decided who would win their matches.
Following the game, Williams admitted that she was a "little rusty".
"My comeback could not have come on a better day and I decided I wanted to do something different let you all be a part of my long journey back", she added.
Williams struggled at times against Bertens but eventually prevailed in two sets
2018 Australian Open champion and second seed Caroline Wozniacki moved into the 3rd round at Indian Wells after defeating Lara Arruabarrena 6-4, 6-1.
The American who already has two Indian Wells title is vying to become the first woman to make it three here clinched her victory on the second match point when Bertens blasted a forehand as she broke the Dutch's serve six times.
Serena boycotted the event until 2015. "Even with the baby, like, a trial run of traveling with the baby and all of this is just so new to me". This is the earliest they will face each other since then.
It was a blistering encounter between the 23-time Grand Slam champion and Bertens, who was a former French Open semi-finalist.
Much like the first set, the second set was a test of wills, with Williams and Bertens trading breaks at times and struggling to hold.
The fourth-seeded Elina Svitolina dropped the first set to Mona Barthel before she charged home to win 2-6 6-1- 6-1.