Nick Kyrgios was penalised a game early in the second set as the volatile Australian went down 7-6 (7/5), 6-2 to Andy Murray in the Stuttgart semi-finals on Saturday.

The cause of his latest meltdown was unclear, with the Australian penalised a point and then a game for unsportsmanlike conduct to trail 1-0 after losing the opening set after 65 minutes.

The incident resulted in a smashed racquet and a few words with a group in the crowd.

After the tournament supervisor was called, Kyrgios merely went through the motions on court as Murray took complete command to secure victory in 93 minutes.

"The second set was not much fun to play, it was not very competitive," Murray said after reaching his 70th career final. "It wasn't really a match.

"He started well but closer to the end he got frustrated. The first set was tough and the end of the first set was the crucial part of the match."

Murray reached his second final of the season after Sydney in January, and will face Matteo Berrettini for the pre-Wimbledon title on Sunday.

Berrettini reached his fourth career final on grass as he overcame German Oscar Otte 7-6 (9/7), 7-6 (7/5). 

The runner-up at Wimbledon last year is competing for the first time since March after undergoing hand surgery. 

He will be seeking his first title since winning Queen's in London last June. 

The world number 10 improved to 27-6 on grass and maintained a perfect 8-0 record in Stuttgart after winning the 2019 edition at the Weissenhof Club.

"It was really tough, but I'm very happy with the result," Berrettini said.  

"I had my operation a few months ago and now I'm back in a final. It feels even better - but the job is not done yet."