PARIS (Reuters) - Former champion Garbine Muguruza suffered a first-round defeat as big names tumbled in the French Open first round on Monday.

The Spaniard, who beat Serena Williams to win the title in 2016, was outplayed 6-1 6-4 by Ukraine's Marta Kostyuk.

Muguruza, the 12th seed, racked up 40 unforced errors as she suffered her earliest ever defeat at Roland Garros.

It was a poor day for former women's Grand Slam champions with Canada's Bianca Andreescu becoming the highest women's seed to fall at the French Open so far, losing a three-hour marathon against Tamara Zidansek.

Sixth seed Andreescu, the 2019 U.S. Open champion, went down 6-7(1) 7-6(2) 9-7 to the Slovenian.

Canadian Andreescu, 20, served for the match at 5-4 in the deciding set but Zidansek broke back before missing a match point of her own in a gripping finale.

Andreescu found herself in trouble again serving at 7-8 and this time was unable to dig her way out of trouble with world number 85 Zidansek winning in three hours and 20 minutes.

While on paper it was a shock, Andreescu was playing at only her third Tour-level claycourt tournament and her lack of matches on the surface ultimately proved decisive.

Her build-up was hardly helped by having to pull out of the Madrid and Rome tournaments after testing positive for COVID-19.

"I think I prepared super, super well for this tournament. That's why to me it's very disappointing, because I thought I could go far," Andreescu, who has struggled with injuries since wining the 2019 U.S. Open, told reporters.

"It sucks right now for me and I can just right now learn from it, because that's what life's all about.

"Hopefully the hard work that I did put in today and over the past couple of weeks will really show for the grass, for the hardcourt season."

Latvia's Jelena Ostapenko, who blasted her way to the French Open title in 2017, also went out, losing 6-4 4-6 6-3 to last year's runner-up Sofia Kenin.

The women's draw lost another big name when 2016 French Open semi-finalist Kiki Bertens fell at the first hurdle, losing 6-1 3-6 6-4 to another Slovenian, Polona Hercog.

Marketa Vondrousova, the 2019 runner-up who is seeded 20th this year, managed to avoid an early exit, battling back from a set down to beat Kaia Kanepi 4-6 6-3 6-0.

Johanna Konta, a semi-finalist in 2019, also crashed out as the 19th seed lost 7-6(5) 6-2 to 54th-ranked Romanian Sorana Cirstea -- ending British hopes in the women's singles.

(Reporting by Martyn Herman; Editing by Christian Radnedge and Pritha Sarkar)