Veteran karate-ka Sandra Sanchez is used to defying the odds, but the thought that she would celebrate a gold medal and a wedding anniversary with her husband-coach on a historic day for the sport they love was beyond even her wildest imagination.

One month shy of her 40th birthday and 15 years older than her youngest competitor at the Tokyo Olympics, Spain's Sanchez was once written off as too old for the highest echelons of the sport - including by her coach-turned-partner Jesus del Moral.

But on a memorable day for karate - the discipline made its Olympics debut on Thursday at Nippon Budokan, the spiritual home of Japanese martial arts - Sanchez took the sport's maiden gold in a tough match against hometown favourite Kiyou Shimizu.

"It's the perfect moment," Sanchez said. "It's Japan, it's the Budokan, it's the (wedding) anniversary. Everything was for this moment."

Sanchez started practising karate at the age of four alongside her older brother and over her long career has won a record 36 medals in the sport's top-billed Karate 1 Premier League, earning her a place in the Guinness World Records.

On Thursday, Sanchez and Shimizu each scored 27.86 in their ranking rounds, underscoring just how close the final would be.

Two years ago in the same spot, the perennial rivals Sanchez and Shimizu had faced off in a rare tie-breaker final at the Karate 1 Premier League. The Japanese took that trophy.

"It was a match that maybe most people thought was lost given the perceived hometown advantage for Shimizu," del Moral told reporters. "But I always said she (Sanchez) can do it. I had faith in her."

Asked how the couple would celebrate the medal and their five-year wedding anniversary, he said jokingly: "I want to invite her to the Olympic Village for dinner."

On a more serious note, he had added that he had another job on Friday to support Team Spain's Damian Quintero in his kata competition.

"Tomorrow evening, we'll celebrate - together," del Moral said.

While her gold medal triumph may be a hard day to top, Sanchez said she was not done with karate yet.

"In three months, there's a world championship," she said, referring to the November event to be held in Dubai.

"After that, I'll decide day by day."