For 19-year-old Eala, the upset third-round victory over the American fifth seed extended her dream run in Miami where she has now toppled two grand slam champions, having beaten 2017 French Open winner Jelena Ostapenko in the second round.
Eala, who has trained at the Rafael Nadal Tennis Academy in Mallorca since she was 13, required a medical timeout for what appeared to be a leg injury but was a force from the baseline against Keys, who reached the Indian Wells semi-finals last week.
"I am just in disbelief," Eala told Tennis Channel.
"I knew I could win from the start but the chances were low given that she is a great player.
"But I think my belief and the trust I had in myself is what pushed me through."
Up next for Eala will be Spanish 10th seed Paula Badosa, a 6-3 7-6 (7-3) winner over Dane Clara Tauson.
In other matches, former champion Iga Swiatek earned a 7-6 (7-2) 6-1 win over Belgian 27th seed Elise Mertens that made the Pole the first player to reach the last 16 of a WTA 1000 event in 25 consecutive appearances.
"I'm happy that I got my level up in the tiebreaker to close it in two sets," Swiatek said.
"Also in the second set I felt like I was playing good, big confidence.
"Yeah, overall I'm happy with the performance and how I worked through some issues."
Up next for Swiatek will be Ukrainian Elina Svitolina, who beat Czech 15th seed Karolina Muchova 6-2 3-6 6-2 and is looking to build on her run to the Indian Wells quarter-finals where she lost to eventual champion Mirra Andreeva.
Andreeva won't be contesting the Miami decider though after the Russian lost 7-6 (7-5) 2-6 6-3 to 17th seed Amanda Anisimova. The American teenager will next face Brit Emma Radacanu.
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