[Note 1] All of his victories were claimed over the Western Front against the Royal Air Force's (RAF) Bomber Command.

On 8 February 1945, Semrau and his crew were killed in action when they were shot down during a daytime maintenance flight by a RAF fighter aircraft.

Semrau was born on 12 November 1915 in Deutsch Eylau, present-day Iława in northern Poland, at the time in the Province of Prussia within the German Empire.

[4] World War II in Europe began on Friday 1 September 1939 when German forces invaded Poland.

Staffel of Zerstörergeschwader 2 (ZG 2—2nd Destroyer Wing) flying the Messerschmitt Bf 109 D single engine fighter.

In March 1940, he transferred to the (Zerstörergruppe) of Kampfgeschwader 30 (KG 30—30th Bomber Wing) where he learned to fly the Junkers Ju 88 multirole combat aircraft.

[5] Following the 1939 aerial Battle of the Heligoland Bight, Royal Air Force (RAF) attacks shifted to the cover of darkness, initiating the Defence of the Reich campaign.

Each sector named a Himmelbett (canopy bed) would direct the night fighter into visual range with target bombers.

Until October 1941, I. Gruppe operated from the Gilze-Rijen Air Base,[8] and commanded by Major Karl Hülshoff.

[1] Semrau claimed two aerial victories on the night of 10/11 February 1941 over two Bristol Blenheim bombers shot down near Feltwell.

[13] Almost a month later, Semrau was awarded the Iron Cross 1st Class (Eisernes Kreuz erster Klasse) on 12 June.

[4] That night, he was credited with the destruction of a Handley Page Halifax bomber near of Finningley, his fifth aerial victory.

[5] Semrau claimed his last long range intruder aerial victory on the night of 20 September when he shot down a Handley Page Hampden bomber 7 kilometers (4.3 miles) northeast of Upper Heyford.

[17] The Battle of the Mediterranean and North African Campaign began in June 1940 with the Italian Empire's entry into the war on the side of the Axis powers.

Gruppe on the night of 9 October over an unidentified four-engine bomber, on a mission to bomb Hanover, 35 kilometers (22 miles) west of Den Haag.

[24] That night, Semrau was credited with three aerial victories over unidentified four-engine bombers in the greater Ruhr area.

[25] On 12 November 1944, Semrau was appointed Geschwaderkommodore (wing commander) of NJG 2, succeeding Günther Radusch.

[9] On the night of 6/7 January, the RAF attacked the German railroad junctions at Hanau and Neuss with over 600 bombers.

[26] Semrau was shot down on 8 February 1945 and killed in action on a factory flight with his crew, Oberfeldwebel Johann Hantusch and Fahnenjunker-Oberfeldwebel Robert Behrens, of Junkers Ju 88 G-6 (Werknummer 620 562 — factory number) when he was intercepted during the landing approach at Fliegerhorst Twente by a Supermarine Spitfire flown by Flight Lieutenant K.S.

[28] According to Spick, Semrau was credited with 46 nocturnal aerial victories claimed in an unknown number of combat missions.