Serena Williams tells how youngest daughter is 'a tough little cookie'

Serena Williams has told how her youngest daughter is "a tough little cookie."

The 23-time Grand Slam tennis champ admitted that when she fell pregnant with her second child, Adira, now 15 months, she was worried she wouldn't love her as much as her first daughter, Olympia, now seven.

"Olympia was a solo child," Serena told People magazine. "She was our star. And I am so obsessed with Olympia. Even when I was pregnant, I was like, 'How do I even love anyone as much as Olympia? It just doesn't work.'"

She continued, "It's so funny how, I don't know, just your heart gets bigger. I was just like, my heart just got bigger."

She added, "Olympia is a very hard act to follow, but I tell you what, Adira has followed it and it's just so crazy. She's amazing. I just can't wait for people to see how amazing she is in her own way. And she's so special and she's so tough. She's just a tough little cookie. And she reminds me a lot of me in that aspect."

Serena is the younger sister of Venus, who is 15 months older than her, so she understands what it's like to be the little one in the family.

"I have a little soft spot for the younger sister, so I'm like, 'You go girl, you can do it too,'" she said.

"I'm like Olympia, 'Make sure you share your toys.' I know what it's like, I have to stick up for Adira."

Serena shares Olympia and Adira with her husband of seven years, Alexis Ohanian.