World No. 2 Aryna Sabalenka suffered a shock straight sets Miami Open quarter-final loss to unseeded 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 No. 4 Caroline Garcia of France in both legs of the "Sunshine Swing."

"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," said the veteran.

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.

Earlier this week, former U.S. Open champion and Canada's world No. 31 Bianca Andreescu suffered two torn ligaments in her left ankle at the Miami Open and is unsure when she will be fit to return to action.

Meanwhile, in the men's field, Italy's Jannik Sinner eased into the semi-finals of the Miami Open after a comfortable 6-3, 6-1 win over Finland's Emil Ruusuvuori on Wednesday.

The 21-year-old Italian has not dropped a set so far in Miami and he coped well with the first break point of the match, holding in the fourth game. 

Sinner has not lost to a player outside the top 50 this year, and he never looked back after that brief threat, breaking the Finn's serve for 3-2 and again to win the set.

In the last four, Sinner will face the winner another quarter-final between world No. 1 Carlos Alcaraz and American Taylor Fritz.

(With input from agencies)