The 22-year-old recovered to serve out the opener at the second attempt, but Wang – ranked two places lower – rediscovered her quality to level, partly fuelled by a disputed line-call in the 10th game.
Raducanu, who had her blood pressure taken on court in a lengthy stoppage, then left for a short break before the decider.
The 2021 US Open champion looked invigorated on her return and quickly moved a double break ahead.
The pair continued to trade breaks before Raducanu reasserted her authority – with the 17th break of the match – to secure a gruelling victory in two hours and 44 minutes.
Raducanu’s relief at coming through the joint longest clay-court match of her career showed as she wearily dropped her racquet to the ground.
“I was just fighting through. It was really difficult,” Raducanu said.
“It didn’t really go away throughout the whole match.
“I’m really happy because it would have been easy to let it drag me down.”
