Antonia Lottner stunned top seed Belinda Bencic to win her first WTA match for 10 months in Switzerland.
The 22-year-old German qualifier, ranked 182 in the world - 160 places below Bencic - triumphed 7-5 6-2 in the first round of the Ladies Open in Lugano on Tuesday.
Lottner will play the Czech Republic's world 101 Kristyna Pliskova after she overcame Ylena In-Albon.
Magdalena Frech lost to Rebecca Peterson 3-6 6-7 (2-7), while Stefanie Voegele beat Pauline Parmentier in straight sets.
Tamara Korpatsch lost to Timea Bacsinszky 6-0 6-2 and Evgeniya Rodina beat 16-year-old Clara Tauson 6-7 (2) 6-2 7-5.
Vera Lapko beat Reka-Luca Jani 6-2 2-6 6-2 and world number 27 Carla Suarez-Navarro lost 3-6 6-3 6-4 to Polona Hercog.
Fiona Ferro beat Mandy Minella while Slovakia's Viktoria Kuzmova triumphed over Giulia Gatto-Monticone.
Iga Swiatek also progressed in straight sets over Katarina Zavatska.
At the Copa Colsanitas in Bogota, Colombia, top seed Jelena Ostapenko was stunned by American lucky loser and world number 205 Kristie Ahn.
The Latvian world number 29 broke her opponent three times in the opening set to gain an early advantage, but Ahn prevailed in a second-set tiebreak to even up the scoreline.
The pair exchanged breaks during a closely fought decider and Ahn saved two match points at 5-4 before triumphing in a mammoth final game, prevailing 2-6 7-6 (5) 7-5 after two-and-a-half hours on court.
Former champion Lara Arruabarrena started her campaign with a win, fighting back from a set down during a rain-interrupted opener to beat Timea Babos 6-7 (7) 6-4 6-1.
After two tight sets, the number 11 seed, who won the title in 2012, cruised to a 5-1 lead in the third before serving out the match after two hours and 15 minutes.
Number seven seed Sara Sorribes Tormo eased past America's Christina McHale 6-4 6-4, setting up a meeting with Ana Bogdan, who overcame French qualifier Chloe Paquet 6-2 6-7 (7) 6-2.
Magda Linette, seeded eighth, rallied from behind to oust Spain's Aliona Bolsova 2-6 7-6 (5) 6-0, while Sara Errani and Bibiane Schoofs also progressed through to the second round.
A match between Shelby Rogers and Astra Sharma was suspended due to darkness, with Sharma leading 6-3 6-7 (6) 5-1.
