Former world number two Petra Kvitova has lauded American tennis superstars Serena and Venus Williams' longevity in professional tennis.
Serena recently lost to Spanish player Garbine Muguruza at the Indian Wells having defeated Belarusian star Victoria Azarenka in the first round.
"I didn't see anything from the last match with Serena played," said Kvitova referring to the win over the Belarusian on Tennis World.
"I don't know. But definitely both of them, they are champions. They did a lot for women's tennis, for sure.
"They are still playing great in that age, which it's nice to see, for sure. They have a great serve, so I think that's still a big weapon for them."
Serena, 37, holds the most Grand Slam titles in singles, doubles, and mixed doubles combined among active players.
She has also won an all-time record of 13 Grand Slam singles titles on hard court.
Her elder sister, Venus, 38, first reached the world number one ranking in February 2002, and she became the first African American woman to do so in the Open Era.
She has won seven Grand Slam singles titles.