Ex-All Black scrumhalf Justin Marshall regards former rival Joost van der Westhuizen as the greatest No.9 in Springbok history.

The legendary scrumhalves, along with Wallaby great George Gregan, were arch rivals for nearly a decade and considered the top three players in the position for the majority of that period.     

The trio have formed a close bond since their playing days and Marshall and Gregan have been dedicated supporters of Van der Westhuizen as he battles motor neuron disease.

Marshall said the 1995 World Cup winner, who played in 89 Springbok Tests and scored 56 tries, is the best scrumhalf South Africa have ever produced.  

"When he was on song he could flip a game and win it almost single-handedly,” Marshall told Fairfax News.

“He was a magic player. One time I remember he made a break through the line, I was defending deeper, and I thought this was my chance to line him up and put him into a hospital ward.

“I launched myself, only for him to chip over my head, pirouette around me and score beside the posts."

Marshall said his respect and admiration for Van der Westhuizen has only increased since the Springbok legend was diagnosed with motor neuron disease last year.  

"He's suffering, and it's really difficult to see the way he is now," he said.

"A degenerative disease like this is pretty awful. You get your head round that first, then do what you can to help out.

"You know he is going to die from it, and die sooner rather than later."

During his last visit to South Africa in February, Marshall watched in awe as Van der Westhuizen auctioned off precious memorabilia to raise funds for his J9 Foundation in aid of motor neuron disease sufferers.    

"To see him giving up the things that meant the most to him to support the fight against motor neuron disease, it was a pretty overwhelming thing," said Marshall.