Mirra Andreeva belts 'Happy Birthday' song to coach Conchita Martinez in Stuttgart

Mirra Andreeva gave coach Conchita Martinez a birthday gift to remember on Wednesday at the Porsche Tennis Grand Prix.

Facing her elder sister, Erika Andreeva, in her tournament debut, Mirra Andreeva advanced to the second round by retirement as her 20-year-old sibling called it quits due to a knee injury when trailing 6-2, 1-0. While the teenager admitted to being "a little bit sad" with how the match ended, earning her first win against her older sister in the process, she perked up when the on-court interviewer brought up former world No. 1 Martinez, who has been coaching Andreeva for nearly a full year.

"I can say for sure that with her it's more relaxed, even though on the court we work hard and sometimes she's a bit too tough with me! That I would like to change, by the way!" the affable 17-year-old joked. "If she wouldn't be that tough with me, I don't think I would play that well right now, but we have an amazing atmosphere in the team, and I hope she'd agree!"

Read more: How Mirra Andreeva's coach Conchita Martinez keeps teen star grounded

That free spirit quickly came out as Andreeva grabbed the microphone, literally, to sing "Happy birthday!" to Martinez, who was simultaneously celebrating her 53rd birthday.

"I really wanna embarrass her a little bit!" the 17-year-old exclaimed before serenading Martinez, with the help of some of the crowd.

The former Wimbledon champion took it in stride, and broke out in a fit of laughter. But when all was said and done, it was Andreeva who was feeling a little sheepish.

Watch the wholesome moment in the video below.

Prior to kicking off her first-ever Stuttgart appearance, Andreeva told reporters that she hoped for "many more years" with Martinez as coach.

But for her sake, Andreeva might hope for fewer sing-alongs.

"Normally I sing better than this, guys -- believe me!" Andreeva later wrote as she reposted the video to her Instagram.