Winston-Salem, NC, USA, 26 August 2016 | AAP

Australian John Millman has caused the biggest upset of his career to reach his first top-tier ATP Tour semifinal.

Millman shocked world No.15 Richard Gasquet 7-5 6-3 in the quarters of the Winston-Salem Open in North Carolina on Thursday (Friday AEST).

The win against the No.1 seed is Millman’s highest-ever ranked scalp and comes just days out from the US Open.

He will now face Spaniard Pablo Carreno Busta, who beat Russian Andrey Kuznetsov.

“It’s been real great to play here all week, it hasn’t been easy,” Millman said.

“Richard is a great player and he makes you work so hard for every point.”

The pair traded breaks of serve throughout the first set before Millman produced a risk-free brand of tennis in the second to take out the match.

Struggling in the hotter conditions, the 28-year-old also took the unusual step of calling for a soft drink during the match for hydration.

“I just needed a little bit of boost and that was Pepsi to give me that ride,” he said.

“It’s been a long few weeks. I was at the Rio Olympics and that physically was really tough.

“You’re buzzing every day to represent your country there and then Cincinnati I played four matches there and long days with the rain.”

Millman has reached 12 second-tier ATP Challengers finals, winning eight titles.