Top-ranked Simona Halep of Romania has said Serena Williams is still the best tennis player in the world.

"I think she should have been actually No. 1 seed in this tournament because she left as No. 1 in the world," the Romanian was quoted by WTA official website as saying on Wednesday, Xinhua reported.

"And to give birth, it's the best thing in the world. It's more than a sport."

Halep took time out to head to the stands and watch the Williams sisters face off for the 29th time in their career in the ongoing Indian Wells WTA tournament.

Serena Williams has not played for a long time after being a mom last year. She lost to her sister Venus in third round match of the event.

"It's always nice to watch them," Halep said. "I love the way that they are motivated and they are still playing at this age, Serena with the kid. So it's a great thing what they do for sport, and it's great that tennis has them.

"It was really fun to come out here and watch the game. And also, you know, I have many things to learn from them. That's why I'm trying just to go in to watch every time I can."

And the 26-year-old, who beat Chinese player Wang Qiang in the fourth round at Indian Wells, added: "I'm the world No.1 in this moment, but I just watched the best player in the world."