US sport outfitter Under Armour, faced with a sales slump, may consider withdrawing from the tennis and fishing markets, the Wall Street Journal reported on Monday.

The US business daily, citing anonymous sources, also wrote that Under Armour is considering the exit while Kip Fulks, 45, one of its co-founders, is taking a sabbatical year. Kip Fulks was regarded as the right-hand man of CEO Kevin Plank, and was the group's strategy consultant before he took time off from the start of the month.

In June, Under Armour appointed Patrick Frisk, former boss of footwear brand Aldo, as COO and deputy chairman of the board of directors, thus taking over a large part of Kip Fulks' functions.

Contacted by the AFP agency, Under Armour has not yet replied.

All of this is happening as Under Armour's share price is struggling on the stock market: since the start of the year, the group has lost nearly 42% of its market value, and at 19.15 GMT on Monday it lost a further 2.92%, falling to $16.97.

Under Armour is the official sponsor of British tennis star Andy Murray and of US tennis player Sloane Stephens, winner of the Women's title at the last US Open championship.

In August, after posting losses for two consecutive quarters, Under Armour announced a restructuring plan featuring a 2% reduction in its total workforce and the closure of a number of factories, and also revised its annual growth forecast downwards.

Once a minor player in the sport apparel and footwear industry, Under Armour took centre stage following the ascendancy of planetary basketball star Stephen Curry.

The brand rode the athleisure wave attracting a whole new customer segment, but its main rivals Nike and Adidas appear to have made up for lost ground, thanks especially to their products and promotional activity targeting Millennials.

Under Armour was founded in 1996, and it is now pinning its hopes on the UAS (Under Armour Sportwear) label, designed by one of the emerging names in fashion, Belgian Tim Coppens, in order for its sales to find new impetus.