In 1910, he won in Hamburg (DSB Congress, Hauptturnier-B). He played in friendly matches: Berlin–Prague (1913), Berlin–Holland (1920), Germany–Netherlands (1922), and Germany–Sweden (1922). Schlage twice won the Berlin City Chess Championship in 1921 and 1926.
He took third place at Hamburg 1921 (21st DSB-Congress, Ehrhardt Post won), tied for 3rd–5th at Bad Oeynhausen 1922 (22nd DSB-Congress, Post won), tied for 11–13th at Berlin 1928 (BSG, Aron Nimzowitsch won), tied for 5–7th at Berlin 1930 (Karl Helling won), tied for 7–8th in the Berlin-ch 1932 (Helling won), tied for 8–9th at Swinemünde 1932 (Gösta Stoltz won), tied for 6–7th in the Berlin-ch 1933 (Berthold Koch and Kurt Richter won), shared 11th at Bad Aachen 1935 (3rd German Championship, Richter won), tied for 3rd–4th at Berlin 1937 (BSG-B, Carlos Guimard and Ludwig Rellstab won), and tied for 4–7th at Krefeld 1938 (Erich Eliskases and Ludwig Engels won).
In 1935, he became Reichstrainer des Großdeutschen Schachbundes (Chief Trainer of the German Chess Federation). Schlage, with Alexander Alekhine and Efim Bogoljubow, trained the German national team for 3rd unofficial Chess Olympiad at Munich 1936. In August 1939, he trained the best young German players (Klaus Junge (15 years old), Wolfgang Unzicker (14), Edith Keller (17), Karl Krbavic (17), Rudolf Kunath (15), etc.) in Fürstenwalde (Jugendschachwoche).
In the film 2001: A Space Odyssey, the HAL 9000 supercomputer is shown soundly defeating the astronaut Frank Poole in a game of chess. The board position and final moves shown replicate a tournament game Schlage won in 1910.
- Poole versus HAL 9000: Roesch vs Willi Schlage, "A Chess Odyssey", Hamburg 1910, DSB-Congress, Hauptturnier-B, Ruy Lopez, Worrall Attack (C86) 0–1
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- Roesch vs Willi Schlage, Hamburg 1910, DSB-Congress, Hauptturnier-B, Ruy Lopez, Worrall Attack (C86) 0–1 at Chessgames.com