Though he learned the game of chess relatively late, at age thirteen, Petrovs made rapid progress.
He won at Helsinki in 1936, and tied for first with Samuel Reshevsky and Salo Flohr at Kemeri in 1937, ahead of Alexander Alekhine, Paul Keres, Endre Steiner, Savielly Tartakower, Reuben Fine, Gideon Ståhlberg and others.
He achieved a particularly brilliant result playing on top board at Buenos Aires: he was undefeated, drawing with world champion Alekhine, former world champion José Raúl Capablanca, and the young superstar Keres, and won against Vladas Mikėnas, Roberto Grau, Tartakower, and Moshe Czerniak.
When Nazi Germany invaded the Soviet Union on 22 June 1941, Petrovs was unable to return to his wife and daughter at home in Latvia.
He remained in Russia and was arrested on 31 August 1942 under Article 58 for criticising decreased living standards in Latvia after the Soviet annexation of 1940.