Latvia's fifth seed Jelena Ostapenko became only the second defending champion to lose in the first round of Roland Garros when she slumped to a shock 7-5, 6-3 defeat by Kateryna Kozlova of Ukraine.
By losing, Ostapenko joins a ignominious group including, as well as Myskina, Steffi Graf, Jennifer Capriati, Svetlana Kuznetsova and Angelique Kerber who all lost as Grand Slam champions on the first run of their defences.
Kozlova, the world number 66, will face either former world number one Victoria Azarenka or Katerina Siniakova of the Czech Republic for a place in the last 32.
Second seed Alexander Zverev got his title bid off to the perfect start with a 6-1, 6-1, 6-2 thrashing of Lithuanian Ricardas Berankis.
Zverev will meet either Czech Jiri Vesely or Serbia's Dusan Lajovic in the second round.
Earlier, fourth seed Grigor Dimitrov reached the second round, even if he wasn't exactly sure who he was facing, while Venus Williams was defeated and Australian firebrand Nick Kyrgios limped out.
Dimitrov had been drawn to face experienced Viktor Troicki in the tournament opener on Philippe Chatrier Court.
However, just before he was due to go on court, Troicki withdrew with a back injury, allowing Egyptian world number 182 Mohamed Safwat to make his Grand Slam debut.
In the process, he became the first Egyptian since Tamer El Sawy at the 1996 US Open to feature in a major.
"I was warming up and my coach said, 'hey look' and we saw up on the board that I was playing a different opponent," said 27-year-old Dimitrov, a former Wimbledon semifinalist, after his 6-1, 6-4, 7-6 (7/1) win.
Seven-time major winner Venus Williams suffered back-to-back opening round exits at the Slams for the first time in her 21-year career.
The ninth-seeded, 37-year-old American slumped to a 6-4, 7-5 defeat to China's world 91 Wang Qiang.
Kyrgios was forced to withdraw after failing to recover from an elbow injury, saying a potential five sets on clay was "too risky".
The 23-year-old was seeded 21 and was due to play compatriot Bernard Tomic in the first round.
"Having consulted with my team and medical experts it is deemed too risky for me to step out and potentially play five sets on clay, especially as I have not played a singles match in nearly two months," Kyrgios wrote on Twitter.
Kyrgios's withdrawal meant that eight places in the first round were opened up to lucky losers.
Women's fourth seed Elina Svitolina, a quarterfinalist in Paris in 2015 and 2017, overcame a first set 1-5 deficit to see off Australia's Ajla Tomljanovic 7-5, 6-3.
Svitolina next faces Slovakia's Viktoria Kuzmova who put out 2010 champion Francesca Schiavone, the 37-year-old Italian who made it through qualifying, 7-6 (7/2), 7-6 (7/2).
Other early winners on Sunday included Estonian 25th seed Anett Kontaveit who downed Madison Brengle of the United States 6-1, 4-6, 6-2.
US Open champion Sloane Stephens, the 10th seed, needed just 49 minutes to breeze past Arantxa Rus 6-2, 6-0.
Results: First round (prefix denotes seeding): Men's singles: Corentin Moutet (FRA) bt Ivo Karlovic (CRO) 7-6 (9/7), 6-2, 7-6 (7/5); Martin Klizan (SVK) bt Laslo Djere (SRB) 7-6 (8/6), 4-6, 7-6 (8/6), 6-1; 32-Gael Monfils (FRA) bt Elliot Benchetrit (FRA) 3-6, 6-1, 6-2, 6-1; 10-Pablo Carreno Busta (ESP) bt Jozef Kovalik (SVK) 4-6, 6-1, 7-5, 7-6 (7/5); Jared Donaldson (USA) bt Nicolas Jarry (CHI) 6-3, 4-6, 3-6, 6-3, 6-4; 4-Grigor Dimitrov (BUL) bt Mohamed Safwat (EGY) 6-1, 6-4, 7-6 (7/1); 19-Kei Nishikori (JPN) bt Maxime Janvier (FRA) 7-6 (7/0), 6-4, 6-3; 15-Lucas Pouille (FRA) bt Daniil Medvedev (RUS) 6-2, 6-3, 6-4; 2-Alexander Zverev (GER) bt Ricardas Berankis (LTU) 6-1, 6-1, 6-2
Women's singles: Kateryna Kozlova (UKR) bt 5-Jelena Ostapenko (LAT) 7-5, 6-3; Ekaterina Makarova (RUS) bt Zheng Saisai (CHN) 6-4, 6-1; 26-Barbora Strycova (CZE) bt Kurumi Nara (JPN) 1-6, 6-3, 6-4; Yulia Putintseva (KAZ) bt 22-Johanna Konta (GBR) 6-4, 6-3; Jennifer Brady (USA) bt Amandine Hesse (FRA) 6-1, 6-1; Petra Martic (CRO) bt Wang Yafan (CHN) 6-2, 6-3; Wang Qiang (CHN) bt 9-Venus Williams (USA) 6-4, 7-5; 4-Elina Svitolina (UKR) bt Ajla Tomljanovic (AUS) 7-5, 6-3; 25-Anett Kontaveit (EST) bt Madison Brengle (USA) 6-1, 4-6, 6-2; 10-Sloane Stephens (USA) bt Arantxa Rus (NED) 6-2, 6-0; 32-Alize Cornet (FRA) bt Sara Errani (ITA) 2-6, 6-2, 6-3.