But, showing no signs of nerves or tension in her first grand slam final, Kenin fought back doggedly to storm through the second set.
The writing was on the wall for Muguruza when she dropped serve for a fourth time, from 40-love up no less, to hand Kenin a 4-2 lead.
Kenin saved break point to hold for 5-2 before Muguruza sadly double-faulted on match point as the American prevailed after two hours and three minutes.
The 21-year-old 14th seed is the youngest player to win the Open since fellow Russian-born American Maria Sharapova in 2008.
Fittingly, her story is not unlike Sharapova’s, with Kenin’s father and coach Alexander fleeing the Soviet Union and taking English classes by day and driving taxis by night in the hope of giving his family a better life.
Almost three decades on and his precociously talented daughter is now the youngest American to crack the world’s top 10 since the great Serena Williams in 1999.
More importantly she’s a grand slam champion after backing up her stirring semi-final win over Australia’s world No.1 Ashleigh Barty with a fearless display of attacking tennis on one of the sport’s biggest stages.
Chasing a third grand slam title of her own, Muguruza was undone by eight double-faults after making the early running on Rod Laver Arena.
The Spaniard broke Kenin in the third game and controlled for much of the opening set.
After missing several chances to forge to a double break, Muguruza, in a worrying sign of what was to come, coughed up successive doubles to allow Kenin back on level terms at 4-4.
Shaking off the disappointment, the former Wimbledon and French Open champion broke straight back before clinching the first set in 53 minutes.
This article originally appeared on 7NEWS