Donna Vekic took full advantage of her late inclusion in the St Petersburg Ladies Trophy field by springing a shock on Tuesday.
Having earned a place in the main draw when Stefanie Voegele withdrew, Vekic - the world number 88 - breezed past Latvia's Jelena Ostapenko, a player 55 places above her in the WTA rankings.
Ostapenko failed to win a game in the first set and threw away a 4-2 lead in the second to go down 6-0 6-4.
Defending champion Roberta Vinci overcame a second-set wobble to beat Timea Babos 6-3 3-6 6-4, while Ana Konjuh bounced back from losing the first set 6-1 against Monica Niculescu to triumph in three sets and set up a meeting with top seed Simona Halep in round two.
Venus Williams will begin her campaign against Kristina Mladenovic, a straight-sets victor over Elise Mertens, while eighth seed Daria Kasatkina beat Belinda Bencic 6-2 7-5.
There were also opening-round wins for Yulia Putintseva, Andrea Petkovic and Alize Cornet.