If to be young is heaven, Sofia Kenin is floating on an untouchable cloud after beating Garbiñe Muguruza in three sets to win the 2020 Australian Open, becoming the eighth first-time women’s champion in the past dozen grand slams. The 21-year-old daughter of Russian emigres, who arrived in the United States with little but hope and two children, is not only the youngest American to win a major since Serena Williams 18 years ago, but the youngest Melbourne champion since Maria Sharapova in 2008.

There were brief moments early in her 4-6, 6-2, 6-2 win at Rod Laver Arena – witnessed by, among many celebrities, the man himself and Margaret Court, the finest Australian player in the history of the game – when Kenin looked as if she might find the occasion too much for her.

Those who know her well talk of a quiet, shy player but on court she has the stamp of a champion, sure of her ability and able to fight her way out of tight corners, which she has had to do several times in the final stages of the tournament, as the matches became tougher.

“This is my first speech, but I’m going to try my best,” she said before going into a blizzard of thank yous that took in everyone from her opponent, the crowd, her team and her parents. “My dream officially came true. I am so emotional. I have worked so hard. These have been the best two weeks of my life.”

Smiling and nursing a glass of champagne more than an hour later, Kenin was a picture of youthful joy and contentment, paying tribute to her Russian roots. “I definitely think [that] helped me,” she said. “I’ve looked up to Maria Sharapova, Anna Kournikova. I followed their matches when I was little. I feel like I got the feisty thing. I saw what it’s like. She won a grand slam at 17, Maria, which I remember watching it on TV. I have part of Russian stuff inside me, the fight and fierceness that I have. Trying just to be confident, do what I do best. And thank you to my parents for giving me the American dream.”

She added: “I’m so proud of myself, my dad, my team, everyone that has been around me. We’ve all worked hard. We’ve been through tough times. We did it. We fought. I’m on cloud nine.”

Muguruza, naturally enough, was downbeat and conceded, “At important moments, I didn’t find my shots and she found hers. I lacked a little bit of energy after so many matches. Physically, it was a tough battle.”

Kenin, who moves up to No 7 in the world and No 1 in her own country, is on the map now, joining a string of prodigies that have begun to make their presence felt at the highest level in recent months and years as Williams wanes and more experienced players fight to fill the vacuum.

“She is not a flash in the pan,” Chris Evert said of Kenin on ESPN. “She is going to be around a long, long time. She can improve, her serve, her volley. But, more importantly, she has the taste of winning a grand slam. There are going to be some great rivalries in the future. We can’t rely on Serena to hold that torch for much longer.”

Muguruza, unseeded and recovering from a virus, beat two top 10 players on the way to the final to confirm the pedigree that had already won her the French Open and Wimbledon, but Kenin showed her respect only in the sense of treating each ball as if it needed to be destroyed. “You’re going to win more finals, for sure,” the Spaniard told her conqueror.

When Muguruza broke early, it seemed Kenin might fold under the pressure such an experienced and fierce competitor can bring to the big occasion. But the American displayed the same tenacity as in her win over Ashleigh Barty, the world No 1, in the semi-finals, hunting down one lost cause after another.

Sofia Kenin stretches for a forehand return. Photograph: Scott Barbour/AAP

Her weapons are not big but they are reliable. Nevertheless, despite taking the Spaniard to deuce in the sixth game, Kenin could not stop her getting to 4-2 with a solid back-hand volley.

Muguruza served for the set after 47 minutes. With two set points in her pocket she wobbled, refusing a volley and making a mess of her tentative forehand from the middle of the court. She was relieved when Kenin went unnecessarily hard at a big winner and found the tramlines.

If Kenin is the game’s Rock of Gibraltar, Muguruza is its Kilimanjaro: daunting but beatable. It took Muguruza five days to conquer the fabled mountain in November, but her final was turned upside down in 20 minutes as Kenin took advantage of her noticeably poor movement and weakened serve to breeze to a 4-1 lead. Serving to stay in the set, Muguruza handed Kenin another set point and she took it to level at one apiece. The fightback was officially up and running.

Muguruza, who had to fight hard to get to the final, looked to be paying the price for that struggle. The physio came on during the interval to check some stiffness in her left hip, but did not treat her. If her problem was more psychosomatic than physical, she was in trouble, continuing to wreck good work in long rallies with careless ground strokes.

She went missing for extended stretches in her semi-final against Simona Halep, yet managed to regroup each time. So it proved in the third set as both players raised their level, very few points given up cheaply.

Following a big serve wide to the net at deuce, Muguruza dollied an easy shot long and double-faulted to go 2-4 down.

With ball in hand, and her head dipping towards the ground in that idiosyncratic way of hers just before her ball-toss, Kenin had the title on her racket. Hitting fiercely and screaming like a banshee, she willed herself to within a game of victory.

Muguruza stepped up to try to repair the damage at 2-5, as the match neared two hours. She found an eighth ace for 30-0, down the ‘T’, but Kenin replied to her second serve with a wicked, unreachable backhand. She responded with another ace – then double-faulted twice for deuce. Another serve floated wide and Kenin cashed in when the second effort dropped mid-court.

Kenin hit long on championship point, kissed the line to get another shot at glory. Muguruza saved her the chore of winning it outright with her third double-fault of the game, her eighth of the match. It was a curious end to an absorbing final.