Defending champion Serena Williams was left frustrated by her performance as she secured a place in the last 16 of the French Open with a 6-4 7-6 (12-10) victory over Kristina Mladenovic.

The top seed, who is aiming for her 22nd grand slam title, looked laboured at times against the French number one in a match interrupted by rain for two-and-a-half hours.

But while the 34-year-old readily admitted that she had not been at her best, she felt that Mladenovic deserved credit for her battling display.

"I feel like I made a tremendous amount of errors," she said. "But, you know, I feel like she kind of forced me to. 

"She forced me to go for it and unfortunately, I wasn't hitting great. It was definitely a challenge for me, obviously. 

"I had opportunities to end it a lot sooner and I didn't, but that's also something that I can try to do, you know, going forward."

Williams went on to identify her inability to convert break points as one of her major frustrations during the round-of-32 clash. 

"I think in general I definitely didn't play my usual game," she said.

"And some of those break points, most of them I should have won and I didn't. 

"Lately I have been really converting those break points, but, you know, I have got to focus on the positives, I guess."

Williams will face Elina Svitolina in the next round after the 18th seed defeated Ana Ivanovic.