Donnachie upset perennial favourite Garrett Reynolds of the United States to take victory with a best run of 84.33 points at the Sydney Olympic Park’s Spotless Stadium.

Sydney-native Lewis Mills impressed the home crowd with a silver medal-winning performance on 84.00 points.

The 20-year-old Mills, competing in his first X Games competition, was the youngest in the field.

He forced team-mate and eight-time gold medallist Reynolds to settle for his first-ever X Games bronze medal.

Today’s other final saw America's Mariah Duran repeat her victory from July's X Games Minneapolis, clinching the gold medal having taken the top spot early.

Her first-run score of 89.00 points held for the rest of the competition.

Brazil’s Leticia Bufoni finished second after posting a solid final-run score of 88.00 points, while Japan’s Aori Nishimura, the silver medallist in Minneapolis, came third with 82.00.

Action in Sydney is due to continue tomorrow and conclude on Sunday (October 21).

Australia is hosting an X Games event for the first time.