His cousin Theodor Alexander Graf von Keller (1857–1918) would later become a famous General der Kavallerie of the Imperial Russian Army.

Also in 1876, Keller volunteered for service with the Serbian Army during the Serbian-Turkish Wars (1876-1878) and was promoted to the rank of lieutenant colonel and distinguished himself in a number of combat operations.

At the conclusion of the war in 1879, he was promoted to colonel and designated an official emissary to Constantinople for the negotiations which determined the borders of the Kingdom of Bulgaria.

After the Russian defeat at the Battle of the Yalu on 1 May, General Kuropatkin decided Keller would replace Lieutenant-General Mikhail Zasulich as commander of the Eastern Detachment of the 2nd Siberian Corps.

[4] Keller's leadership skills quickly made a difference in restoring the confidence of the soldiers who had been severely mauled by General Kuroki Tamemoto's 1st Japanese Army.

[4] On 31 July, Keller established a strong defensive position north of Motien Pass to block the Japanese advance on Liaoyang.