NAIROBI, Kenya (AP) — Olympic 800-meter champion and world-record holder David Rudisha withdrew from the world championships on Monday after straining a thigh muscle in a training session in Kenya.

The double world and Olympic champion said an MRI scan on the right-leg injury showed it would not heal in time for the championships, which open at London's 2012 Olympic Stadium on Friday. The 800 heats are on Saturday.

The injury happened last week, but Kenyan team officials gave Rudisha as long as possible to see if there was any chance of him recovering in time to defend his world title. The team leaves for London on Tuesday.

"The injury occurred in the last 200 meters of my last training session," Rudisha said. "I'm disappointed for not defending my title and running for my country but I have to focus on recovery to prolong my career."

Rudisha announced his withdrawal on Twitter , saying it was "sad and disappointing." Rudisha's tweet was accompanied by a photo of his injured leg with strapping on it.

The 28-year-old Kenyan was set to return to the scene of his greatest triumph, when he won Olympic gold and broke his own world record in a breathtaking, wire-to-wire victory at the 2012 London Games.

Rudisha's time of 1 minute, 40.91 seconds was the first time anyone had gone under 1:41 in the 800.

Rudisha retained his Olympic title at the Rio de Janeiro Games last year and also claimed world championships gold in 2011 and 2015. This will be the second worlds he has missed because of injury after having to skip the 2013 edition in Moscow with a knee problem.

Rudisha is arguably the sport's greatest 800-meter runner, running the three fastest times over two laps and six of the eight fastest times.

He is the second defending champion from Kenya to miss the worlds in London after 400-meter hurdles title holder Nicholas Bett withdrew because of injury last week.