Re: What player on the tour is the biggest fighter?
I would agree that Serena and Maria are incredibly mentally tough and hard to beat. They just have that aura, they dominate the opponent with their personality and always have the possibility of pulling out great shots when they need them to stave off defeat/win a match. In this they are like all great champions. Serena, like Martina Navratilova at her greatest, is a very intimidating court presence that literally scares opponents into defeat sometimes.
Looking back over the last 30 years, two other retired players also come to mind when discussing mental toughness and I am not quite sure who corresponds to them now in terms of their fighting abilities.
Chris Evert was the most calm player ever in a crisis - she was totally unflappable, nerves of steel, every opponent had to beat her, she never lost to anyone. Who is her best equivalent in the modern game?
Arantxa Sanchez Vicario was the most combative and competitive player on a point by point basis - she literally refused to cede any ball, ran down everything and in the process drove opponents nuts. Is there anyone around now who would be prepared to cover so much court just to win one point time after time?