World number one Naomi Osaka was just two points from a sensational Roland Garros exit on Tuesday before securing her place in the second round where she was joined by defending champion Simona Halep who also needed three sets to survive.

Osaka, the reigning champion of both the U.S. Open and Australian Open, battled back from a set and a breakdown to defeat Slovakia's Anna Karolina Schmiedlova 0-6, 7-6 (7/4), 6-1.

However, she came within a whisker of becoming only the second top seed to lose in the first round in Paris in the modern era after German Angelique Kerber in 2017.

"This is the most nervous I have ever been my entire life during a match. You could see that in the first set. I was literally not hitting any balls on the court," Osaka explained.

Romania's Simona Halep said she was "confident" for her French Open title defense after an ultimately comfortable first-round victory on Tuesday despite a second-set blip against Australian Ajla Tomljanovic.

The Romanian was pegged back on Court Philippe Chatrier but raced through the decider to win 6-2, 3-6, 6-1.

China's Wang Qiang got past compatriot Zheng Saisai in straight sets in the women's singles first round.

Zheng easily conceded the first set 6-1 despite managing a break in the fifth game, before making adjustments in the followed-up set to stay ahead at 5-4. Then Wang steadied her nerves to bag three games in a row and secure her win with a precise backhand.

Fellow Chinese player Zhu Lin failed in her maiden Roland Garros main draw appearance with a 6-1, 6-1 loss to Romania's Irina-Camelia Begu.

The longest match of 2019 tournament

Alexander Zverev survived a bruising four-hour battle to reach the French Open second round on Tuesday. German fifth seed Zverev battled past Australia's John Millman 7-6 (7/4), 6-3, 2-6, 6-7 (5/7), 6-3.

"John is a tough player who beat Federer so I knew it would be difficult today," said Zverev, bidding to become the first German man to win the Roland Garros title since Henner Henkel in 1937.

"It was very windy today so that made the conditions very difficult. It was a match against a great opponent."

Roland Garros' oldest matchup

Ivo Karlovic made a mockery of his 40 years to beat relative spring chicken, 37-year-old Feliciano Lopez in the oldest ever men's matchup at Roland Garros on Tuesday.

Croatian Karlovic, the tallest player on the tour at 6ft 11in (2.11m), defeated Spain's Lopez 7-6 (7/4), 7-5, 6-7 (7/9), 7-5.

World number 94 Karlovic fired 35 aces and 69 winners past his 108th-ranked opponent, having wasted two match points in the third set.

(With inputs from Xinhua, AFP)