Women's Tennis Association (WTA) has insisted that it has "zero tolerance for racism" after Chinese tennis player Zhang Shuai broke down in tears and retired from the match at the Hungarian Grand Prix in Budapest on Wednesday.

During a match against local opponent Amarissa Toth, Zhang hit a crosscourt forehand that appeared to land on the line but was called out by the line judge. The chair umpire then stepped down to take a look at the mark and confirmed the ball had landed outside the line. 

Zhang was incensed by the call and asked to speak with the tournament supervisor. The match continued for one more point, but the disagreement over the disputed call continued before Toth walked up to the mark and used her shoes to erase it. The Chinese world number 28 looked visibly distressed during the changeover, and a physio was called to check on her before she opted to retire while trailing 6-5 in the opening set of their round of 32 match in Budapest. 

The home crowd jeered Zhang during the incident, while Toth shook hands with her before putting her arms up in celebration.

 "Thousands of cases like this happen all over the world. In this situation, it is the chair umpire who counts, who has awarded the point," the tournament's communications chief Erik Siklos told state news agency MTI.

Later in the day, the Hungarian Grand Prix defended Toth by posting a comment on its official account on Facebook.  

"The Hungarian tennis player in the first WTA match of her life may not have behaved in every situation, but she did nothing that could be described as a lack of integrity," it read. "And there is no deflection, no misunderstanding. The Chinese are manipulating the world with a manipulative video." 

The Hungarian's controversial move was swiftly condemned by fellow players on social media. 

"Absolutely disgusting behavior," Australia's Ajla Tomljanovic wrote on Twitter. "Shuai is a better person than a lot of us for shaking the ref and that girl's hand." 

Australian doubles player Ellen Perez said Toth had lost the respect of her peers. "I'm actually shook by the level of disrespect from this girl... If I see this girl tomorrow I will tell her how disgusted I am." 

Recent Wimbledon finalist Ons Jabeur also weighed in, tweeting, "All the support to Shuai. This is not acceptable." Former world No. 1 Victoria Azarenka added, "That's another level of unsportsmanlike conduct." 

Greek star Maria Sakkari went as far as saying on Twitter, "Zhang Shuai is the nicest player on the tour! That Toth girl should be banned from the tour."

As the situation grew increasingly severe, the WTA released a statement, saying "The WTA has zero tolerance for racism in any form or context. The unfortunate incident that took place at the Hungarian Grand Prix and subsequent posts are being reviewed and will be addressed." 

Toth, who initially defended her decision to erase the mark, issued an apology on Wednesday.  

"I am extremely sorry for what happened, I respect Zhang Shuai as a player and as a person. It was never my intention to disrespect, hurt or upset anybody, let alone Zhang Shuai," she said. 

"I do realize I shouldn't have celebrated the way I celebrated after the match and I'm sorry for that. I was acting in the heat of the match and got caught up by my emotions and the moment. I focused on tennis, I didn’t want to win like that." 

Toth noted that she was hoping to speak with Zhang in the future to express her regret. 

(With input from agencies)