Japan's Naomi Osaka battled back from a set and a break down to reach the Roland Garros second round on Tuesday while Grand Slam under-achiever Alexander Zverev needed four hours to get through his opener.
Top seed Osaka, bidding to add the French Open to her US and Australian Open titles, defeated 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 Angelique Kerber in 2017.
"I was really close, so it hurts I lost so many chances," said Schmiedlova.
"Maybe I felt like it was against the world number one. It was not easy in my head."
World number 90 Schmiedlova, who hadn't won a match at the tournament since 2014, twice served for victory in the 10th and 12th games of the second set.
At one stage, she was just two points away from a famous victory.
But Osaka, who had arrived in Paris still feeling the effects of a hand injury suffered in Rome, battled back to set-up a second round duel with former world number one and two-time major winner Victoria Azarenka.
Azarenka, a semi-finalist in 2013, reached the second round for the first time since 2015 with a 6-4, 7-6 (7/4) win over 2017 champion Jelena Ostapenko.
German fifth seed Zverev, a quarter-finalist in 2018, battled past Australia's John Millman 7-6 (7/4), 6-3, 2-6, 6-7 (5/7), 6-3 in a shade over four hours.
The champion in Geneva last weekend fired 57 winners past world number 56 Millman who stunned Roger Federer at the US Open last year.
However, he also committed 73 unforced errors on a blustery day in the French capital.
"John is a tough player 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.
Next up for Zverev is Swedish qualifier Mikael Ymer, the world number 148 of Ethiopian origin, who marked his Grand Slam debut with a 6-0, 6-3, 7-6 (7/5) win over Slovenia's Blaz Rola.
Argentine eighth seed Juan Martin del Potro, a semi-finalist in 2009 and 2018, made the second round with a 3-6, 6-2, 6-1, 6-4 win over Chilean world number 58 Nicolas Jarry.
Canada's Bianca Andreescu, the surprise champion at Indian Wells, marked her Roland Garros debut by beating Marie Bouzkova of the Czech Republic 5-7, 6-4, 6-4.
Croatian Karlovic, the tallest player on the tour at 6ft 11in (2.11m), defeated Lopez 7-6 (7/4), 7-5, 6-7 (7/9), 7-5.
In the late evening clash on Tuesday, Reigning champion Simona Halep started her title defence with an ultimately comfortable 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. Halep, seeded third, will face Poland's Magda Linette in the second round.