Marion Bartoli believes "superhero" Serena Williams is already close to her best at Wimbledon.

Williams missed last year's grass-court grand slam as she was pregnant with her first child, but the American has progressed to the semi-finals at the All England Club on her return.

And Bartoli has been stunned by the level of the 23-time major champion's play, claiming she is already showing the form that saw her reach at least the last four at 10 consecutive grand slams prior to her lengthy lay-off.

"Serena's barely played any tennis and now she's come back and she is already playing at this level," said Bartoli. "But she's in another category, she's a superhero. She can turn up and win grand slams."

"I've been commentating on all Serena's matches and I think she's very close to her best level. She's maybe lacking a little bit of speed side to side, and maybe she's slightly less dominant with the return of service, but these are small things."

"To me, it's scary how close she is to being back to her best level."

Williams faces Julia Goerges in the semifinals on Thursday.