No matter how much success she has had over the years, and to this day, Serena Williams professes to deal with doubts.

The American began this French Open with a straight-set victory over an overmatched foe in Court Philippe Chatrier on Monday.

Williams has something significant at stake over the coming two weeks: a chance to equal the record of 24 grand slam singles titles.

After she played OK in the opening set, then was terrific in the second, of a 7-6 (7-2) 6-0 win over 102nd-ranked American Kristie Ahn, Williams – so used to being questioned about Margaret Court – was asked on this day about Rafael Nadal and Roger Federer.

“I don’t get involved in the ‘greatest’ talk. You know, the greatest for me is, and will always be, Jesus, so I’m going to leave it at that,” Williams said.

But she didn’t leave it at that.

“Rafa, I’m obviously a huge fan of his. I always have been. But it’s like you can’t compare two people that are equally great. Roger, I mean, he’s Roger Federer. I think that says enough,” Williams said.

“So, you know, it’s like I don’t understand why people want to pit, ‘Who’s this? Who’s that?’ They both have spectacular careers that 99 per cent of people can only dream of and … every single credit and every single thing that they get, they absolutely deserve it.

“I’m a big fan of both, to be honest.”

Ahn faced Williams in the first round of the recent US Open – and lost.

So imagine this (bad) luck of the draw: Ahn is now the only player to face Williams in the first round at two consecutive grand slam tournaments.

Of note: Williams is 75-1 in openers at majors.

“I mean, I laughed,” Ahn said. “I mean, what are the odds?”

Yet she was up to the task for most of the 72-minute opening set at Roland Garros, twice leading it by a break.

And then?

What changed for Williams?

“I just need to play with more confidence, like I’m Serena,” she said.

Ahn said trying to stop a confident Williams was like “trying to push a runaway train in the opposite direction”.

That job in the next round will go to Tsvetana Pironkova, in a rematch with Williams after the quarter-final clash in New York.

Among the other key results on Monday were opening round defeats to last year’s runner-up Marketa Vondrousova, three-time grand slam champion Angelique Kerber and American 12th seed Madison Key.

Those through to the second round included Dutch fifth seed Kiki Bertens, seventh-seeded Petra Kvitova, Spanish 11th seed Garbine Muguruza and 2012 French Open finalist Sara Errani.