On 22 August, Kirui took gold over his teammate Emmanuel Mutai who took silver at the 2009 World Championships in Athletics in Berlin at a record time of 2:06:54.

Immediately after that failure, he started training with the Italian Coach Renato Canova, moving from Kapsabet to Iten, together with Jonathan Maiyo.

Training with Canova's group, he was able to recruit the best shape, winning in Daegu his second title in World Championships.

[4] He was given the wildcard spot on the Kenyan marathon team as the defending champion at the 2011 World Championships in Athletics.

Kirui ran a time of 2:07:38 beating out fellow countryman Vincent Kipruto and Ethiopian Feyisa Lilesa.

This made Kirui the third man to defend the marathon world title, following in the steps of Abel Antón and Jaouad Gharib.

[12][13] Kirui along with fellow Seventh-day Adventists Priscah Jeptoo and Amos Tirop Matui founded Better Living Marathon.