Four-time Olympic champion Michael Johnson believes Usain Bolt can run 100m in 9.4 seconds - but only if the Jamaican refines his sprinting style.
The 25-year-old broke his own world record with a time of 9.58 secs at the 2009 World Championships in Berlin.
Johnson said: "Bolt could absolutely run faster, but it would take a tremendous commitment on his part to do a complete overhaul of his technique."
But the American still thinks Bolt is favourite to retain his Olympic titles.
"He could probably run 9.4 seconds," added 44-year-old Johnson, who won gold in Barcelona in 1992, did the 200m and 400m double in Atlanta four years later, and claimed another gold in the 2000 Sydney Games.
"But he would have to do some major training and adjustments in the way that he runs."
Bolt, who will also be defending his 200m and 4x100m Olympic titles in London, announced on Thursday he was going to withdraw from the Diamond League meeting on 20 July after suffering a "slight" injury problem.
The Monaco event had been scheduled to be his final race before London.
Johnson told Laureus.com that Bolt was "not a very clean sprinter" from a biomechanical standpoint, compared to America's Tyson Gay or Jamaican Asafa Powell.
Johnson cited analysis conducted at the Michael Johnson Performance Center in Dallas.
While being very impressed by Bolt, he thinks the outcome of the men's 100m in London is far from settled.
"Yohan Blake showed he will take advantage when Bolt does not perform at his best," said Johnson.
"Bolt now has a legitimate challenger and will need to be at his best in London to defend his Olympic title."
Blake beat Bolt at the Jamaican Olympic trials in both the 100m and 200m, running 9.75 secs to Bolt's 9.86 secs in the shorter distance.
But Johnson believes Bolt ultimately controls his own destiny.
"I think Usain can do whatever he wants to do, he's got that type of talent and he wants to win three gold medals again," said Johnson.
"I would say if he gets to the starting line healthy, at his best, everyone else at their best, he wins every time. I mean he's just that good."