They are not playing for their country. Therefore the players can choose any partner they want. There are other tournaments like Davis cup and Olympics. In these tournaments players represent their country and the partners are from their own country.
Thereof, how many sets are in mixed doubles?
In this regard, who receives the serve in tennis doubles?
The same player must serve the entire game. ADVERTISEMENT So if you serve the first game, the opposing team will serve the second game, your partner will serve the third game, the partner on the opposing team will serve the fourth game and you will serve again on the fifth game, and so on.