World number two Aryna Sabalenka suffered a shock straight sets Miami Open quarter-final loss to resurgent Romanian Sorana Cirstea, who reached her first WTA 1000 semi-final in a decade.

Cirstea, ranked 74th in the world, triumphed 6-4, 6-4 against the Australian Open champion to secure her first WTA 1000 semi-final since Toronto in 2013.

The 32-year-old is enjoying an upturn in form since taking on Swede Thomas Johansson as her coach, reaching the quarter-finals at Indian Wells and beating world number four Caroline Garcia of France in both legs of the "Sunshine Swing."

In her first meeting with the big-serving Belarusian, Cirstea looked comfortable and composed as she broke Sabalenka's first service game.

The 24-year-old from Minsk, who is now based in Miami, broke back to make it 4-4 but Cirstea swiftly got back in front and held to win take the opening set with her opponent looking visibly irritated and frustrated as she laboured in the early afternoon heat and humidity.

Again, Cirstea broke on her opponent's first serve game and then at 5-4 up, she survived two break points before a superb ace down the middle gave her match point, which she converted when Sabalenka went long.

The veteran reacted with a broad smile and a fist pump and conceded she was surprised by the victory.

"I am a bit speechless," she said. "I came out hoping that it would be a really tough match. I tried to hold my ground. I'm very happy with my performance today, it is a bit unexpected to be honest."

"I had confidence in my game. I knew I was playing well, and I knew that if I come out swinging and being aggressive, I would have my chance," she added.

"I'm very happy to get the win against Aryna, because she has been playing great lately. She's probably one of the two or three best at the moment. So this gives me a lot of confidence."

Sabalenka, whose year began with victory at the Australia Open before she reached the final in Indian Wells, losing to Elena Rybakina, said she had struggled with the heat.

"It definitely wasn't my best match. I was struggling a lot with the conditions, like heat. I felt like balls were flying too much and I couldn't find control," she said.

"I was just trying to do my best 'til the last point. I just couldn't adjust to these conditions unfortunately.

"The first three months of the year were fantastic for me. I'm just proud of the consistency of my game and I have to keep working, keep improving," said the Belarusian.

In the last four, Cirstea will face the winner of the quarter-final between Ekaterina Alexandrova and Petra Kvitova which was postponed due to heavy rain and will be moved to Thursday.