Mid 1930s, probably after 1932.
Only thirty years earlier, in 1901, here is how England's Charlotte Cooper dressed to play tennis at Wimbledon.
But in 1932, Alice Marble made history by wearing shorts in professional play. Here's a beautiful Alfred Eisenstaedt photograph of Marble from 1937:
Pictorial Review usually provided wonderful copy for their patterns and this one is no exception:
Straight from the French resorts comes this ultra-smart sports costume. The tailored blouse may be worn with high or low neckline, and long or or short cuff-trimmed sleeves. The shorts have a buttoned placket closing and inserted pockets. The sash slips through straps and ties jauntily at the side.The espadrilles in the pattern illustration, while very fashionable resort wear at this time seem a bit odd for sportswear, but really, it's more about the look than the actual sport. The fashion editors at Pictorial really seemed to know what they were talking about. The odd little cap in the Pictorial illustration shows up in several of Lartigue's photographs of Renee Perle, most of which were taken in the South of France in the early 1930s, so there may have been a fad for them.