The 18-year-old has quickly become the story of the Championships, having only played one WTA Tour level match before this tournament.
On Saturday, she showed she belongs on the big stage with a 6-3 7-5 victory.
Raducanu, who is waiting for her A-Level results, passed this particular test in fine style, with a run of eight successive games laying the foundations of a memorable afternoon.
She said in her on-court interview: “Honestly, I am so speechless right now...I am so so grateful for all the support I had today."
As well as a sparkling attacking display on Saturday, she also showed fighting spirit and tenacity in the second set, claiming the win on her third match point.
Raducanu's headteacher Alan Blount describes her as hardworking and diligent, 'exactly the same as she is on the tennis court'
Raducanu is the last British woman in Wimbledon singles. She is also the youngest British woman to reach round three since the late Elena Baltacha in 2002.
“Who would have thought, it is funny because at the start when I was packing to come into the bubble, my parents were like, ‘Aren’t you packing too many sets of match kit?’," she said.
“I think I am going to have to do some laundry tonight, but I think they have a laundry service at the hotel, so I am all good.”
In a precursor for what was to come, she forced break points in the opening game of the match, which Cirstea saved, before holding to love in her first service game.
The Romanian is a top-50 player, beating Victoria Azarenka in the previous round, and flexed her muscles by claiming an early break to go 3-1 up after a smart backhand down the line.
If she thought she had broken the spirit of Raducanu, she was very wrong and the Brit proceeded to rattle off eight successive games which put her in a winning position.
She immediately broke back and then held to love to make it 3-3 and there was no looking back as she demolished Cirstea with an array of attacking winners, sealing the first set with a brilliant backhand defensive lob.
That carried over into the second set, quickly racing into a 3-0 lead, and at 0-40 on the Cirstea service game, the finish line was in sight.
But tennis is a funny sport when it comes to momentum and a mixture of some Raducanu errors and some big serves allowed Cirstea to wriggle out of her predicament and changed the complexion of the second set.
Cirstea then broke Raducunu and held again to draw level at 3-3, which is when the drama began.
Raducanu showed guts to come from 0-30 down to hold serve and to stop the rot before a tension-filled 14-minute game on Cirstea’s delivery.
Raducanu had her chances, with five break points and nine deuces, but she could not get over the line.
Cirstea’s serve had come under threat the whole afternoon, though, and it was the same again, with Raducunu this time clinching the match at the third time of asking when her opponent dumped a backhand into the net.