She managed to get through to the third round at Indian Wells earlier this month on her return to action, but was no match for an in-form Stearns in Miami.
Boulter was broken three times in a row in the first set and although she showed a glimpse of promise in the second, her 23-year-old opponent soon took control to progress.
“Katie is a great player, it is always tough playing her,” said Stearns.
“I knew I had to stay on it and if I gave her anything she would take it.”
