Serena Williams believes Monday's much-anticipated showdown against Maria Sharapova at the French Open will provide an assessment of her progress since returning to tennis.

The long-time rivals will go head-to-head in the fourth round at Roland Garros after both recorded comfortable victories on Saturday.

Williams - playing in a grand slam for the first time since giving birth last September - triumphed 6-3 6-4 against German Julia Goerges after 75 minutes on court.

However, the three-time champion insists Sharapova, who dropped just three games as she brushed aside Karolina Pliskova, is the favourite to progress to the quarter-finals.

Champion in 2012 and 2014, Russian Sharapova was refused a wild card for last year's tournament after serving 15-month ban for failing a drugs test, but has looked in ominous form on clay in the French capital.

"We are both on a comeback for two totally different reasons, and she's been on her journey for over a year and I just started mine a couple months ago," Williams told the media after her routine win.

"Quite frankly, she's probably a favourite in this match, for sure.

"You know, she's been playing for over a year now. I've just started, so I'm just really trying to get my bearings and trying to feel out where I am and see where I can go.

"But I think this will be another test. I think this is just one of her best surfaces, and she always does really, really well here.

"This will be a good opportunity for me [to] kind of to see where I am and just hopefully continue to go forward."

The rivalry between the pair was heightened after Sharapova claimed in her autobiography - released last year - that she heard Williams crying in the dressing room after losing to the Russian in the 2004 Wimbledon final.

While admitting to wearing her heart on her sleeve, Williams told reporters she felt such information should not be made public.

"I think the book was 100 per cent hearsay, at least all the stuff I read and the quotes that I read, which was a little bit disappointing," the 23-time grand-slam champion said.

"You know, I have cried in the locker room many times after a loss, and that's what I have seen a lot of people do. I think it's normal.

"I think if anything, it shows the passion and the desire and the will that you have to want to go out there and do the best.

"It's a Wimbledon final, you know. I think it would be more shocking if I wasn't in tears. And I am emotional and I do have emotions and I wear them on my sleeve.

"You know, I'm human. So for me, I think it's totally normal. I think what happens there should definitely maybe stay there and not necessarily talked about it in a not-so-positive way in a book."