"It has a little bit to do with not having a formal education" - When Serena Williams reacted to Martina Hingis' 'big mouth' claims about her family

At the 1999 US Open Serena Williams had a fiery response for Martina Hingis, who called her and her family "big-mouthed."

Hingis, who was the World No. 1 at the time, was asked during the tournament about Richard Williams' claims that both his daughters would contest the title clash. She responded by claiming that the family had a big mouth and that they talked a lot.

"They always have a big mouth. They always talk a lot. It happened before. It's going to happen again. So I don't really worry about that," Hingis said.

Serena Williams was then asked about the Swiss' comments after she beat Jelena Kostanic in the second round. The American claimed that Hingis' statements and her willingness to always speak her mind had a little bit to do with not having a formal education.

"That's just her problem. She's always been the type of person that, she says things, just speaks her mind. I guess it has a little bit to do with not having a formal education. But you just have to somehow just think more, you have to use your brain a little more in the tennis world. She does that well on the court," Williams said.

"She's No. 1. She's doing very well. But, I mean, I just don't have anything negative to say about anyone. It just literally does not pay to say anything negative and bad about anyone. So... That's her opinion and she's entitled to her -- everyone is entitled to their own separate opinion. Obviously she's No. 1 so she can say whatever she would like to say. I personally don't think my mouth is big, if you're looking at it," Williams added.

Serena Williams and Martina Hingis went on to face each other in the 1999 US Open final, with the former winning 6-3, 7-6(4).

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Serena Williams and Martina Hingis played each other 13 times on tour

Serena Williams at the 2011 US Open

Serena Williams and Martina Hingis locked horns on 13 occasions, with the American leading 7-6 in the head-to-head between the two. Their first meeting came in the quarterfinals of the 1998 Lipton Championships, where Hingis won 6-3, 1-6, 7-6(4).

Williams and Hingis faced each another in two finals. The former came out on top in the 1999 US Open title clash, while the latter won the Du Maurier Open (now known as the Canadian Open) in 2000.

The last match between the two came in the quarterfinals of the NASDAQ-100 Open in 2002, with Williams winning 6-4, 6-0.