The match was close early on - Kerber went up 3-1, Cirstea won three games to lead 4-3, Kerber held for 4-all. But from there the No.28-seeded Cirstea completely took over, winning eight games in a row - and losing just seven points in those eight games - to finish off the No.6-seeded Kerber, 64 60.
"Today I played quite solid and was consistent from the beginning until the end," Cirstea said. "I was trying to do the plan my coach gave me and I think everything worked like it was supposed to.
"To be honest, I didn't have any expectations coming into this tournament because I've never played that well in Miami. But I've been fighting the best I could and working my way through every single match, and it's working. When you work so hard on everything, it's nice to see the results coming - it gets you going and gives you motivation to keep going and keep working hard and everything else.
"I love this sport, and I love moments like this."
Cirstea's win over the World No.6 was her ninth Top 10 win. She has had Top 10 wins on all three surfaces - hard, clay and grass - and she has even had Top 10 wins on all three colors of clay, one on green (beating Anna Chakvetadze at Charleston in 2008), two on red (beating Caroline Wozniacki and Jelena Jankovic at the 2009 French Open) and one on blue (Marion Bartoli at Madrid last year).
Next up for Cirstea will be the No.22-seeded Jankovic, who won a gripping two-setter against No.11 seed Nadia Petrova, 76(7) 64. The first set alone lasted an hour and 20 minutes, with Jankovic saving a set point down 6-5 in the tie-break; the second set lasted almost an hour too - 57 minutes.
"It was a battle in the first set, but thankfully I was able to get that first set in the tie-breaker," Jankovic said. "The beginning was really tight but once we got to the second set I was more comfortable, and I think she was getting a little bit tired, and I took advantage of that. It was a very tough match."
Cirstea leads Jankovic in their head-to-head, 2-1. Cirstea won marathons in their first two meetings on clay, but in their last meeting - which was on hardcourts - Jankovic cruised in straight sets.
And those weren't the only upsets of the day - No.21 seed Klara Zakopalova surprised No.14 seed Maria Kirilenko, 62 76(4). Things got complicated in the second set, as after Zakopalova built a 62 42 lead, Kirilenko won three games in a row and even held double set point as Zakopalova fell behind 15-40 in that 4-5 service game; but the Czech hung on and eventually closed it out in the tie-break.
Next up for Zakopalova is No.3 seed Maria Sharapova, who swatted aside an early threat from No.29 seed Elena Vesnina, 64 62. Vesnina came out firing, breaking serve for a 3-1 lead in the first set - but Sharapova caught fire from there, winning 11 of the next 14 games to advance in straight sets.
"It wasn't easy for the both of us with the heat and the wind," Sharapova said. "It was just one of those days where you know you might not be playing your best tennis - I certainly wasn't today.
"But I got through, and I'm onto the next one."
Sharapova leads Zakopalova in their head-to-head, 4-1. Zakopalova won their first encounter when Sharapova was only 15 - Sharapova has won all four meetings since, including in Doha this year.