Nike and Tag Heuer have become the first major sponsors to cut ties with Maria Sharapova after the tennis star revealed she had tested positive for the banned substance meldonium.

The five-time grand slam winner announced on Monday that she failed the drug test after she was knocked out of the Australian Open in January. She has been provisionally suspended from the sport by the International Tennis Federation.

Nike, Sharapova’s most high-profile sponsor, said it would be suspending their relationship. “We are saddened and surprised by the news about Maria Sharapova,” it said in a statement.

“We have decided to suspend our relationship with Maria while the investigation continues. We will continue to monitor the situation.”

Swiss luxury watchmaker Tag Heuer, which has had a relationship with Sharapova since 2004, said it had stopped negotiations with the tennis player, whose previous contract had ended on 31 December.

“We had been in talks to extend our collaboration,” a spokeswoman said. “In view of the current situation, the brand has decided not to renew the contract with Sharapova.”

In 2015, Forbes estimated that the tennis player, the world’s highest-paid female athlete for the 11th year running, had earned £16m in endorsements. Her lucrative eight-year contract with Nike was signed in 2010 and was worth almost £50m.

Other endorsements for the former world No 1 include a three-year deal with Porsche, Avon, Evian and Head tennis rackets.

The star also owns her own high-end gummy sweet brand, Sugarpova, launched in 2012 and now sold in 22 countries, and had plans to launch chocolate in a collaboration with Baron Chocolates this year.

On Tuesday, Russia’s tennis chief Shamil Tarpischev called Sharapova’s failed drug test “a load of nonsense” and said he expected she would still compete at the 2016 Olympics in Brazil this summer.

“The sportsmen take what they are given by the physiotherapists and by the doctors,” he said. “I think Sharapova will play at the Olympics, however, we will need to see how this will develop.”

Russia is currently barred from international competition after a major doping scandal involving its athletes last year, but hopes to meet anti-doping standards set by the International Association of Athletics Federations in time for the Olympics, though many international observers believe this is unrealistic.

Sharapova has also said she hopes she can eventually return to the sport, despite being plagued by injuries.

“I let my fans down, I let the sport down that I have been playing since the age of four and I love so deeply,” she said in a press conference at a Los Angeles hotel on Monday.

“I know with this I face consequences. I don’t want to end my career this way and I really hope I will be given another chance to play this game.”

The drug Sharapova was using was placed on the banned list by the World Anti-Doping Agency from 1 January this year because of “evidence of its use by athletes with the intention of enhancing performance”. Sharapova claimed she had taken meldonium legally for years for health reasons.

Steve Simon, chief executive of the Women’s Tennis Association, called Sharapova “a woman of great integrity”.

“Nevertheless, as Maria acknowledged, it is every player’s responsibility to know what they put in their body and to know if it is permissible,” he said. “The WTA will support the decisions reached through this process.”

(Guardian service)