Kristie Ahn

Aiming to finally secure a 24th grand slam title and draw level with Margaret Court's record tally, Williams twice dropped serve in a competitive opening set that spanned 74 minutes. 

Ahn had also provided solid early resistance when the two Americans met in the first round at the recent US Open, though eventually lost on that occasion in straight sets. 

The world number 102 suffered a similar fate in the French capital, simply unable to cope against an opponent who moved through the gears to cruise through to round two.

Williams had appeared to warm quickly to her task despite the cool temperatures in Paris, winning her first service game to love in a hurry.

However, the sixth seed stuttered afterwards and was broken next time around, Ahn edging ahead at the sixth attempt to grab a lead she maintained through to the eighth game. 

Williams levelled at 4-4 – much to her obvious delight as she yelled out when finally clinching a game that spanned 12 minutes and 42 seconds – and while broken immediately afterwards, Ahn was crucially unable to serve out for the set. 

The tie-break ultimately proved a one-sided affair, the three-time champion on clay clinching it in style with an ace. 

Playing in a far more aggressive manner, Williams dominated in the second to set up a clash with another familiar foe in Tsvetana Pironkova, who she beat in the last eight at Flushing Meadows. 

Data slam: Williams was surprisingly tepid in her approach in the early going. Unable to assert any dominance, she committed 28 unforced errors in a first set that could quite easily have gone Ahn's way. Yet from sluggish beginnings, she grew into the match and the second was far more straightforward, aided by four aces and 11 winners.

WINNERS/UNFORCED ERRORS

Williams – 26/36
Ahn – 13/27

ACES/DOUBLE FAULTS

Williams – 11/5
Ahn – 1/4

BREAK POINTS WON

Williams – 5/14
Ahn – 2/8

Williams, who turned 39 on Saturday, took her time to warm up on a cool Monday in Paris, Kristie Ahn making life particularly difficult for the sixth seed in a well-contested opening set.

The American duo had also met in the opening round of the US Open and - as was the case in New York - the favourite eventually prevailed in straight sets,meaning a clash with another familiar foe next.

Pironkova threatened to cause an upset when they did battle in the quarter-finals at Flushing Meadows earlier this month, though faded down the stretch to lose in three.

Still, Williams - chasing a 24th slam to move level with Margaret Court's record - expects another tough test from the Bulgarian, who defeated Andrea Petkovic 6-3 6-3.

"She's playing well, but I am too. I'm ready to play her. She'll be ready to play me," the three-time French Open champion said in her post-match press conference. 

"It will be a long match, she will get a lot of balls back, but so am I. I'll be ready."

Williams revealed she was a little flat against Ahn in a first set that went to a tie break, before a more positive approach allowed her to breeze through the second without dropping a game.

"The biggest difference was just confidence. I just need to play with more confidence, like I'm Serena," she said. "So that was it. I just started playing like that. And I love the clay and I started playing like it, opening the court and moving and sliding."

SUCCESS FOR SEEDS ON DAY TWO

Three other top-10 seeds at the tournament avoided early exits. Elina Svitolina, Kiki Bertens and Petra Kvitova prevailing, though none of the trio had it all their own way.

Bertens was in danger of going home early when the fifth seed fell a set behind but rallied impressively to see off Katarina Zavatska 2-6 6-2 6-0.

Seventh seed Kvitova overcame Oceane Dodin 6-3 7-5, though admitted afterwards her opponent had made her work for the win.

"You know, she didn't make that many mistakes and it was really a tough one," Kvitova - a semi-finalist at Roland Garros in 2012 - told the media. "I just really tried to stay there mentally strong and wait for the chance to break her and serve well."

Meanwhile, Svitolina, the third seed, triumphed 7-6 (7-2) 6-4 against Russian Varvara Gracheva.

KEYS LOST IN FRENCH CAPITAL, KERBER CRASHES OUT

Madison Keys, the 12th seed, was on the wrong end of an upset, the American beaten in straight sets by Zhang Shuai.

Prior to that match on the same court, 15th seed Marketa Vondrousova was crushed by Iga Swiatek of Poland, winning just three games in a surprisingly lopsided contest that spanned just 63 minutes.

Angelique Kerber also suffered a shock exit, the three-time major winner going down 6-3 6-3 to world number 102 Kaja Juvan.

Karolina Muchova was also dumped out, going down to Christina McHale, but 2016 champion Garbine Muguruza avoided a similar fate against Tamara Zidansek, overcoming a one-set deficit to eventually prevail 5-7 6-4 8-6.