Tennis players wear two pairs of socks for the extra cushioning and comfort they gain over the course of matches and tournaments. This helps to prevent blisters and heat spots on their feet.
Players like Roger Federer and Rafael Nadal are known to wear two pairs of socks for this exact reason. Plus, it's simply more comfortable than one pair.
Most tennis socks, even the ones professional tennis players use, are not made with quality, thick yarns. Therefore, some of the top guys reinforce their socks by adding a second pair.
But you can get socks that are thick enough to wear one pair at a time. Brands like us, Clay Active, make our socks in Australia wear we use higher quality cotton, polyester and nylon. That means that the sock is naturally thicker, while having the benefits of a breathable, ventilated professional tennis sock.