A pair of major upsets early on day one of the French Open have blown the bottom half of the women's draw wide open at Roland Garros as Madrid champion Ons Jabeur and 2016 champion Garbine Muguruza were sent packing.

Muguruza has recently appeared out of sorts, with the world number 10 racking up early losses in Madrid, Rome and Rabat, but appeared to be in good touch early in her opening round match with veteran Estonian Kaia Kanepi and took the first set 6-2.

But the ever-dangerous Kanepi, who made the quarter-finals at this year's Australian Open roared back into the match to take it 2-6, 6-3, 6-4 as a frustrated Muguruza slapped the court with her racquet.

Kanepi, who has twice made the final eight at Roland Garros — the last time in 2012 — and won the girl's singles back in 2001 simply showed too much class for Muguruza as she out-hit her, cranking winners from both side in the later sets.

For the 36-year-old Kanepi, the win opens up her part of the draw which includes fellow Estonian, fifth-seed Anett Kontaveit.

Asked how she turned the match around Kanepi said: "I didn't change much, I just made more balls and my serve was better in the second and third sets."

French failure 'difficult to take' for Jabeur

Her dismissal of Muguruza followed the tournament opening with a shock on Court Phillipe-Chartrier as sixth-seed Jabeur, who has been one of the form players on tour this year, was ousted in three sets by Poland's Magda Linette.

Jabeur became the first major casualty when she crashed out with a 3-6, 7-6(7/4), 7-5 loss to the world number 52.

Jabeur, seen as one of the big threats to world number one Iga Świątek at the tournament, came to Paris at a career-high ranking and with a tour-leading 17 wins on the surface this season.

"But that's what sport is like and you need to be smart enough to move forward and get back on court. So maybe it was a good thing to lose today.

"I would rather say this and be really tough with myself than waste all the good energy that I got from Madrid and Rome."

With her win in Madrid and runner-up finishes in Rome and Charleston, it appeared Jabeur would have an easy outing against Linette when she made a strong start.

The 27-year-old suffered an early break of serve before breezing through the opening set in 37 minutes before Linette raised her game.

Linette, 30, had only defeated Jabeur in the first of their four meetings back in 2013 but she staged an inspired fightback in the second set, saving four break points to level the match in the tiebreak.

As drops of rain fell on the main showcourt, Jabeur's level also dipped and her unforced errors climbed and she made her frustration evident by kicking balls away after losing points.

Linette, who lost to Jabeur in the third round last year, went up 4-2 in the third set but saw her advantage slip away when Jabeur broke her serve back to stay alive.

But the Pole sealed the match when the Tunisian wasted a 40-0 lead in the 12th game and found the net on match point to be broken for the second time in the set.

"I lost to her last year here, and then this year in Miami, and so I kind of knew what to expect," Linette said.

Kubler the only winner as Aussies struggle

Right at the end of a long, largely wretched day, Australian tennis has found its saviour on a shocking Sunday at the French Open.

Step forward never-say-die qualifier Jason Kubler, who finally put a first-round victory on the board at 9.40pm at Roland Garros, more than 10-and-a-half hours after play on a brutally hard opening day began.

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The first four Aussies in action — Chris O'Connell, Alexei Popyrin, Astra Sharma and Thanasi Kokkinakis — had already bitten the red dust.

Then Kubler, the man perhaps least fancied to negotiate his first-round tie, stepped up in the darkness — with the floodlights shining on the ominously numbered Court 13 — and saved the day.

The one-time wonder boy of Australian tennis — who had battled through three tough qualifying matches to get to the main draw — defeated streetwise American Denis Kudla 7-6(7/5), 7-6(7/2), 7-6(7/3) to become the first of the 11-strong contingent to make it through.

Sydney wildcard O'Connell had kicked off the singles challenge on a humid morning with high hopes, only to depart Court 8 wholly deflated by a 2-6, 4-6, 7(7/5)-6, 1-6 loss to Slovenian Aljaz Bedene.

Next to fall was last year's "big thing", Popyrin, whose 2022 struggles continued at the hands of Fabio Fognini, the erratic 34-year-old Italian swashbuckler who found the odd touch of his old magic as he gave the young Sydneysider a bit of a lesson 6-4, 7-5, 6-4.

Perth's Australian number two Sharma, the first woman in action, came agonisingly close to victory over Varvara Gracheva, standing just two points away from winning when a startling pass from the Russian kick-started her revival to win 4-6, 6-4, 7-5.

Sharma was so shocked by Gracheva's winner that she actually applauded her in the heat of battle.

Kokkinakis got involved in a real slugfest with Spanish clay-court specialist Albert Ramos-Vinolas, and looked shattered to go down 4-6, 6-4, 4-6, 6-7(7/5) in a see-saw four-setter.

The pain was obvious for the Adelaide man, who missed out on a second-round date with Spain's new teenage comet Carlos Alcaraz, who just got better and better as he pulverised Argentine Juan Ignacio Londero 6-4, 6-2, 6-0.

But former junior world number one Kubler — who'd saved a match point in his second round qualifier against Radu Albot — simply would not be denied as night fell, proving just too teak-tough in each of three tiebreaks that he dominated.

He had two match points denied by the American before he hammered a glorious winning backhand down the line in front of a by-now sparse crowd after more than three-and-a-quarter hours, letting out a primal roar.

ABC/Wires