Only Steffi Graf has won more grand slams in the modern era than Williams with the German claiming 22 during her illustrious career, but one former tennis great believes that Serena wouldn't have had it all her own way in the 1990s.

"When Serena is at the top, it's impossible -- nobody can stop her," four-time grand slam winner Arantxa Sanchez-Vicario told CNN's Open Court show. "You can see the clear domination and I think, for me, and our era, that was impossible.

"The winner at every tournament had to beat three or four of the top players to win the tournament -- now this isn't happening."

Sanchez-Vicario competed in a golden era for women's tennis when the Spaniard and Graf competed against the likes of Monica Seles, Martina Navratilova, Gabriela Sabatini, Lindsay Davenport and Martina Hingis.

Now 44, she believes the dominance of the 34-year-old Serena and the lack of any real challengers has meant less drama in the women's game.

Williams had appeared unbeatable before her shock defeat by Roberta Vinci in the semifinal of the 2015 U.S. Open.

She was then beaten by Germany's Angelique Kerber at the Australian Open earlier this year.

Sanchez-Vicario, who won four of her seven meetings against Williams, including a last-16 victory at the 1998 French Open, says the American is only vulnerable when she drops to a level her rivals can reach.

"If Serena drops her game then maybe somebody could stop her," Sanchez Vicario said.

"It happened at the Australian Open with Kerber. Maybe the pressure that she was going to win the four grand slams was too much for her.

" And Vinci beat her at the U.S. Open but it's very unusual and i think it's a big gap between her and the next players and the next players.

"I would like to see a bit more mobility and better matches all the time rather than only watch the semifinal or finals but that's how the game is.

"Serena is clearly the 100% favorite to win everything."

(CNN)