When did Anton Chekhov begin to focus more on serious short stories?

Anton Chekhov began to focus on serious short stories in 1888.1

With the publication of his long story "Steppe" in the literary review Severny vestnik that year, he turned away from the comic fiction that had characterized his earlier career. This shift was marked by a significant change in his creative output: he moved from producing over a hundred items annually in 1886 and 1887 to focusing on quality over quantity, publishing only 10 short stories in 1888. While humor remained an underlying element in his work, his writing became more serious in conception, leading to profound studies such as "A Dreary Story" (1889).1