He attracted the attention of some of the leading figures French racing and became closely associated with the Chantilly trainers William Cunnington and Frank Carter.
Le Ksar failed in The Derby despite Semblat's confidence: he claimed to have made a careful study of the course "with maps and a book" and to have "already won the race on paper".
By the end of the war he had won two Arc de Triomphes with Djebel and Ardan and trained the stayer Marsyas to the first of four consecutive wins in the Prix du Cadran.
This success was achieved despite wartime restrictions which saw the closure of Longchamp, France's most important racecourse, as well as strict limits imposed both on travel and on the amount of grain used for the horses' feed.
Semblat's training regime involved giving his charges a great deal of walking exercise and ensuring that each horse had a dedicated groom.
[13] The working relationship between Semblat and Boussac eventually deteriorated and in 1954 the trainer was dismissed and replaced by Charlie Elliott, the man he had supplanted fourteen years earlier.