"All the mums out there that had a tough pregnancy and have to come back and try to be fierce, in a middle of everything," she told journalist Jon Wertheim, "that's what this represents. You can't beat a cat suit, right?"

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The last time she had appeared in a grand slam tournament was at the Australian Open in 2017, when she won it while pregnant with daughter Alexis Olympia.

This time there was a little rust to shake off, but Williams was too strong, too good, simply too Serena for her opponent.

It was a match-up that, in some respects, made a mockery of statistics. The American started the match ranked 9,999th in the race to the annual season-ender. Pliskova sits 52nd.

But in the data that counts, Williams is a giant of the tennis court. Her haul of 23 grand slam singles crowns beats Steffi Graf's total by one, and is just one shy of Margaret Court's record 24.

Coming into this tournament her career prizemoney stood at more than $US84 million. That she is currently ranked 451 in the WTA rankings is an irrelevance; more pertinent is that she finished 2017 ranked 22, having played only two events all year.

In every category, Williams' achievements dwarfed her opponent's, bar one: Pliskova holds the WTA record for aces in a single match: 31 at the 2016 Australian Open.

She cracked 15 on Court Philippe Chatrier on Tuesday, but even then saw 13 fly past her as the American gave as good as she got.

"I don't know how many aces there were," Williams smiled courtside after the match. "But I think money goes to charity, so donate! I think we raised some real money."

Then she laughed before making her way to the exit, stopping for selfies and to sign autographs for an appreciative French crowd.

Next up for the three-times champion is 17th seed Barty.

It won't be an encounter Barty will relish. She could be considered the most unlucky of seeds in a grand slam, for there can never have been a more dangerous floater in the draw.

French Open organisers stick rigidly to the WTA rankings, so would not seed Serena. That is already beginning to look like a mistake Wimbledon – which allows itself some 'wriggle room' – would be wise to avoid repeating.