A relieved Serena Williams is anticipating another tricky day at the office when she meets Kiki Bertens in the French Open semi-finals.

Reigning Roland Garros champion Williams was well below her best in Thursday's last-eight clash with Yulia Putintseva, as she was forced to fight back from a set and a break down to eventually prevail 5-7 6-4 6-1.

The unseeded Bertens, meanwhile, registered a 12th consecutive victory in style as she dispatched Timea Bacsinszky 7-5 6-2.

Bertens has now beaten four seeds on her way to the semis, and Williams is wary of the in-form 24-year-old.

"She's been playing really well," said the world number one.

"I saw a little bit of her match, we kind of finished around the same time so I didn't get to see all of it, but I saw some of the highlights. She's a really sweet girl and it's always good to see someone like that do well."

World number 58 Bertens performed admirably in losing 7-6 (7-5) 6-3 to Williams in the second round of last year's US Open, and the top seed went on to offer further words of praise for her opponent.

"I remember her being just really tough, and, you know, giving a good, tough performance. I was happy to get through that match," she added.

"Obviously she has a big serve and a big forehand. She really moves the ball around well, so she does everything the best she can."

On her below-par showing against Putintseva, Williams continued: "I just was not playing my best. I kept missing, you know, just misfiring. Honestly, at one point I didn't see the light at the end of the tunnel.

"I don't know. I guess I was not the most positive mentally, but obviously I didn't want to stop."