The American, who won the 200m and 400m double in the 1996 Olympic Games in Atlanta, is also of the view that Bolt's compatriot Yohan Blake, who also bettered his 19.30 one-time world's best with a 19.26 run at a Diamond League meet in Brussels a few weeks ago, will have to do it again before he is considered a serious threat to the world record holder's dominance.

"I haven't gotten to see the race yet, but only Usain (Bolt) has run faster so it was a very impressive time," said Johnson.

When asked which of the Jamaicans stand a better chance of dipping below 19 seconds, Johnson, 44, reasoned that despite his untidy running style, which sees him rocking from side to side, Bolt is more equipped to break the barrier.

"I think it's Usain. We have to wait and see what's going to happen with Yohan, if he's going to do that again or if he's going to focus more on the 100m," Johnson noted.

"Usain's run 19.3 and 19.19 - I'm certain he can go under 19 seconds," the American added.

Johnson is still the only athlete to win gold in both the 200m and 400m at the same Olympic Games, after his exploits in Atlanta, and is also the only man to successfully defend his 400m Olympic title.

His eight gold medals at the World Championships leaves him tied with fellow American Carl Lewis as the athletes with the most gold medals in history.

He is responsible for 13 of the top 100 times in the 200m and 27 of the top 100 times in the 400m - including 14 of the top 25.