I suggest you accept « Switzerland » for Martina Hingis, since she grew up in that country since the age of 8 and is a Swiss citizen.
You should also accept « UK » for « United Kingdom », as in other quizzes, and « USA » for « United States » (doesn't work for Arthur Ashe).
For Lendl and Navratilova, you should also accept USA, since they are now US citizens.
Same for Monica Seles. I would accept Yugoslavia and USA, but she never played (or lived) with Serbian nationality.
Martina Hingis has always played as a Swiss citizen