Sabalenka was twice a set and a break up on veteran Cirstea, who is retiring at the end of the season.
She also broke the 36-year-old when Cirstea served for the match at 5-4 in the third set – only to immediately hand the advantage back in the next game.
Sabalenka’s serve – the most improved part of her game and the key to her rise to the top of the rankings – was broken six times throughout the match.
She made 72% of her first serves but won just 56% of points behind it and could only take 11 of 26 points behind her second serve.
Sabalenka was left slapping at a ball in irritation after losing five games in a row to concede the second set and go an immediate break down in the decider.
She responded in the next game, but her service games always felt shaky, and a wide forehand handed Cirstea the break lead at 3-2.
Sabalenka called for the trainer and had her back stretched out at 4-3 down. She then looked to be mounting a comeback after stopping Cirstea serving out the match at the first time of asking.
However, Cirstea returned brilliantly to restore her lead and converted her first match point thanks to a long Sabalenka return.
Earlier, defending champion and home favourite Jasmine Paolini was beaten 4-6 7-6 (7-3) 6-3 by Belgium’s Elise Mertens.