'Impossible to believe': Tennis rocked by sudden death of Davis Cup great, 52

The tennis world is mourning the death of Russian tennis great Alexander Volkov. He was 52.

Volkov made it as high as World No.14 throughout his career and helped Russia make the Davis Cup final in 1994 and 1995.

He won three ATP singles titles in an 11-year career from 1988-1998.

Russian Tennis Federation boss Yevgeny Kafelnikov reacted with shock.

“It's impossible to believe!,” Kafelnikov said on Twitter.

"I will always remember your cheerful smile and our matches for the national team in the Davis Cup. Rest in peace, my friend.”

Marat Safin leads tributes

Russian grand slam winner Marat Safin, who Volkov coached, told RBC: "The man who did a lot for tennis helped me a lot.”

"Thanks to him, I became the first [World No.1 from Russia] so this is a person who is very close to me in heart, in spirit.

“Unfortunately, this happens in life.”

Volkov made the round of 16 at Wimbledon four times and reached the US Open semi-finals in 1983 where he lost to Pete Sampras.

He beat Pat Rafter and Michael Stich as he led Russia to the 1994 Davis Cup final, where they lost to Sweden.