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Sabalenka, who lost in the quarter-finals of the Madrid Open last month, was defeated 2-6 6-3 7-5 by Cirstea.

Sabalenka looked to be on her way to a routine victory when breaks in the third and seventh games saw her win the first set.

Sabalenka also got on top of Cirstea's serve at the start of the second set, although the Romanian responded with three breaks of her own to force a decider.

The pair exchanged breaks to start the third before Cirstea, who is retiring at the end of the year, moved ahead in the fifth game.

Sabalenka got back on serve to level the set at 5-5, but Cirstea fired back with another break and sealed victory with her first match point.

Sabalenka conceded that she was physically hampered during the match, but she was was quick to praise Cirstea for her triumph also.

"Yeah, I feel like I didn't play well from the beginning till the end. I started really well, but then I kind of dropped the level," Sabalenka said.

"It felt like my body was limiting me from performing on the highest level.

"I'd say that it's probably like my lower back, connected to the hip, which is kind of like limiting me from the full rotation.

"She [Cirstea] stepped in and played incredible tennis. [She] didn't really give me much opportunities. But I guess we never lose; we only learn, so it's OK.”

At 36 years and 28 days old, Cirstea became the oldest player to defeat the world No.1 on clay, and also claim a comeback win against the top ranked player in the WTA rankings and claim a maiden triumph against the world No.1.

Sabalenka's defeat was just her third of 2026, having completed the Sunshine Double with victories at Indian Wells and the Miami Open in March.

She also won the Brisbane International in January and lost the Australian Open final to Elena Rybakina, but her attention has now turned to Roland-Garros.

"I guess we're just going to have some days off," Sabalenka said when asked about her fitness ahead of the second grand slam of the season.

"We're going to spend it on recovery. That's the plan I believe for now."

Meanwhile, defending champion and home favourite Jasmine Paolini also lost in three sets to Elise Mertens.

Mertens won 4-6 7-6 (7-5) 6-3 in two hours and 44 minutes and will face either Mirra Andreeva or Viktorija Golubic in the next round.