Williams, who was watched by her sister, Serena, had too much guile for the Japanese world number 90 who won their last meeting at the 2016 Indian Wells tournament in the United States.
“The first set was tricky,” said Williams. “She played well – amazing defence and offence but I was trying to control the match which I managed to do and so that feels good.”
In January Williams reached the final of the Australian Open where she lost to her younger sister who promptly announced she would quit the circuit as she was expecting her first child.
“I’m not quite sure when Serena came into watch me,” added Williams. “I’m sure she feels the excitement of being at a major. But not having the pressure to play and prepare must be an interesting feeling for her. She knows exactly what it’s like out there.”