MELBOURNE — America's Sofia Kenin stunned two-time Major champion Garbine Muguruza to win the Australian Open in her first Grand Slam final on Saturday, becoming the youngest player to lift the trophy in 12 years.

Kenin, 21, showed all her trademark aggression as she fought back from a set down to win 4-6, 6-2, 6-2 in 2hrs 3mins.

The 14th seed, who will now jump as high as seventh in the world and usurp Serena Williams as America's No. 1, was in tears at the end and headed straight for her father Alexander, who is also her coach.

It was the final surprise in a tournament of upsets, where Williams went out in the third round and Kenin upset Australia's world number one Ashleigh Barty in the semi-finals.

"My dream officially came true, I can't describe this feeling," said Kenin. "It's amazing, dreams come true. If you have dreams, go for it, it's going to come true."

Spain's Muguruza, 26, unseeded for the first time at a Slam since 2014, broke in the third game of the first set at Rod Laver Arena, where the roof was closed for rain.

Moscow-born Kenin, who also ended the fairytale run of 15-year-old Coco Gauff on her way to the final, bounced her American stars-and-stripes racquet on the hardcourt in irritation.

The powerful-striking Muguruza, who won just one match in the last six months of 2019 but found form at last in Melbourne, held her own serve for a 3-1 first-set lead and was in control.

The highlight of a compelling set came when they were locked at 30-30 on Muguruza's serve. A 23-shot rally culminated in the Venezuelan-born Spaniard clinching a hard-fought point with a forehand volley at the net.

Muguruza, who climbed Mount Kilimanjaro in the off-season to help clear her head of tennis, stubbornly held for 4-2.

Kenin demonstrated the fight that has become her calling card by saving four game points, then pulled level 4-4 when Muguruza produced two double-faults in a row.

The Kenin comeback was shortlived, Muguruza breaking back immediately and taking the set in 52 minutes when the young American planted her forehand out.

But the aggressive Kenin upped the ante in the second set, breaking her more experienced opponent in the fourth game and easily holding to sprint into a 4-1 lead.

Kenin, who won their only previous encounter in three sets, grabbed the second set in an emphatic 32 minutes. A rattled Muguruza was seen briefly by a physio for what appeared to be a lower-back problem.

Into the deciding set and the gutsy Kenin saved three game points in a pivotal fifth game, tossing the ball back over her head by way of celebration.

She was gifted the break for a crucial 4-2 lead when the fading Muguruza again double-faulted, to sighs from Rod Laver Arena.

And Kenin was gifted the title when Muguruza double-faulted once again — her eighth — on the American's second championship point.

At 21 years and 80 days, Kenin is 22 days younger than Japan's Naomi Osaka when she won the title last year. She is the youngest champion since Maria Sharapova won aged 20 in 2008. — AFP