AUSTRALIAN wildcard Jason Kubler can barely believe it after continuing his extraordinary return from the tennis wilderness with a watershed first-round US Open win over Spanish star Roberto Bautista-Agut.

In a memorable and long overdue main-draw debut for the former teenage prodigy, Kubler defied searing heat to oust the 19th seed 6-3 6-3 6-4 in the first big shock of the tournament.

“It was unreal. It’s the biggest win of my career,” Kubler said after sealing victory with a signature backhand pass down the line.

Ranked 945th in the world this time last year, Kubler’s stop-start career appeared over after six knee surgeries and medical advice to only ever play on clay courts.

But after cracking the top 100 for the first time last month, the 25-year-old Queenslander is now also savouring his maiden victory at a major. “It’s crazy to think just how far it’s been in just under 12 months,” Kubler said.

The one-time world No. 1 junior missed eight years of grand slam action before being granted an Australian Open wildcard in January and then successfully qualifying at Wimbledon.

“It was good to have Wimbledon just because it gives me confidence that I did qualify,” Kubler said.

“If I’d got a wildcard into Wimbledon and lost first round, then it’s like you’ve never really earnt your way into a main draw.

“So it’s good to have that just to give you the belief.” Kubler’s reward for his latest milestone moment is a second-round clash on Wednesday with 20-year-old American Taylor Fritz, a five-set comeback winner of Mischa Zverev.

Finally feeling injury free, Kubler is confident his battered body can handle the physical grind of back-to-back grand slam matches for the first time.

“That’s probably the best best-of-five-setter I’ve had,” he said. “I’ve only had three or four of them before, but each of them I was cramping or I was pretty tired.

“But that one was pretty good and now I get the day off. I think I’ll be fine.”