Stuttgart Open organisers are investigating claims made by Nick Kyrgios that he was racially abused at the tournament.

Kyrgios was defeated in the semi-finals by Andy Murray in straight sets on Saturday, with the former given a penalty by the umpire for smashing his racket and arguing with the crowd.

The Australian took to Instagram afterwards to say he was subjected to racial abuse by a spectator and called a "little black sheep".

"When is this going to stop? Dealing with racial slurs from the crowd?" he wrote.

"I understand that my behaviour isn't the best all the time – but 'you little black sheep', 'shut up and play' – little comments like this are not acceptable.

"When I retaliate to the crowd, I get penalised. This is messed up."

The tournament organisers responded on Sunday with a statement that read: "We stand for creating an inclusive environment for all players, staff members and visitors where discrimination of any kind is not tolerated.

"This attitude is lived by all people involved and responsible. These fundamental values are as important as values like fairness, tolerance and team spirit to us. Thus no discriminating actions by the spectators are accepted.

"We have expressed our regret towards Nick Kyrgios and his team and assured that any kind of discrimination is unacceptable. The incident is currently under investigation."