Emma Raducanu was not able to see out victory against Dayana Yastremska at the Slovenia Open as her opponent retired when two points from defeat.
Raducanu, returning to action after a first-round loss at the US Open, where she had been the defending champion, looked set to ease past Yastremska.
The Briton did advance to the second round, but only courtesy of a premature conclusion to the match.
Raducanu led 6-2 5-3 and was 30-0 up in her own service game when Yastremska retired, complaining of a wrist injury.
Yastremska has previously been accused of gamesmanship for similar antics; of her now six career mid-match retirements, all have come with her a break down and about to lose, with four coming in a potentially decisive game.
Her decision to quit at this year's Australian Open was particularly unpopular, with Madison Brengle on the brink of inflicting a third-set bagel.
However, Raducanu did not look to criticise Yastremska after this latest contentious finale.
"Obviously no one wants to win by retirement," the top seed said. "But Dayana is an extremely ferocious competitor, so I knew it was going to be a tough match going in."
Fellow seeds Beatriz Haddad Maia, Ekaterina Alexandrova and Anastasia Potapova all also progressed in Portoroz.
At the Chennai Open, only eighth seed Chloe Paquet was subject to a scalp against Karman Kaur Thandi, with Rebecca Peterson and Eugenie Bouchard among the other victors.
Siegemund won the first set on a tie break, but Kazakhstan representative Rybakina stormed back to win the second before winning a tense 10-8 tie break in the crucial third set, surviving a match point in the process.
The world number 25's victory sets up a second round meeting with Tereza Martincova, who overcame a poor first set to beat Harriet Dart 0-6 6-4 6-1.
At the Chennai Open, top seed Alison Riske-Amritraj was beaten in straight sets in an upset defeat to world number 147 Anastasia Gasanova.
There were no such upsets for the second and third seeds though, as Varvara Gracheva and Magda Linette both cruised to victory to clinch their safe passage to the second round.
Elsewhere, Katie Swan beat Arianne Hartono 6-1 6-2 while India's number one singles player Ankita Raina was knocked out of her home tournament by Tatjana Maria.
The Kazakhstan representative broke Bogdan five times while saving all eight break points to seal a 6-1 6-1 triumph and reach a first final since her SW19 victory over Ons Jabeur in July.
In the final, Rybakina will face Katerina Siniakova after she overcame Anna-Lena Friedsam.
Siniakova stormed to the first set, but the second was a much closer affair and the world number 82 watched five match points come and go before she finally clinched a 6-1 7-5 success.
At the Chennai Open, Magda Linette will play Linda Fruhvirtova in Sunday's decider after coming through semi-final encounters in differing fashion.
Third seed Linette raced out to a 3-0 lead in the first set over Katie Swan before the Brit called a medical timeout after just 16 minutes of play, eventually retiring with illness.
Fruhvirtova's semi-final was a much more gruelling affair, as the 17-year-old came from a set down to overcome Nadia Podoroska 5-7 6-2 6-4 and reach her first WTA final.
The Czech secured the third singles triumph of her career – and first since prevailing at Bastad in 2017 – as she recovered from losing the opening set to deny Wimbledon champion Rybakina 6-7 (4-7) 7-6 (7-5) 6-4.
Siniakova – also the doubles world number one – continued her rich vein of form, having captured her third grand slam of 2022 alongside Barbara Krejcikova at the US Open last week.
The world number 82 had come through two matches on Saturday to reach the championship match in Portoroz, and was slow out of the blocks as she fell 5-0 behind in the opening set. Although Siniakova fought back to force the tie-break, Rybakina held her nerve by winning four of the last five points to draw first blood.
The Wimbledon champion, who hit 43 winners, appeared to be closing in on victory in her third final of the year as she broke for 4-3 in the second set. However, Siniakova broke back immediately before eventually forcing a decider.
Both players exchanged breaks early on in the third, but Rybakina's increasing unforced error tally (66) eventually culminated in her opponent claiming another crucial one for 5-4, before serving it out to end the drought.
Over at the Chennai Open, teenager Linda Fruhvirtova also enjoyed a comeback victory as she denied Magda Linette 4-6 6-3 6-4 to land her maiden WTA title.
Aged 17 years and 141 days, Fruhvirtova became the youngest winner on the WTA Tour this season, and is set to climb into the world's top 100 for the first time.
In a tight opening set, Linette broke for 5-4 before holding to love to strike first. A solitary break also proved crucial in the second set; the Czech profiting on this occasion to ensure the final went to a decider.
Linette, who won just two out of nine break points, established early control as she surged into a 4-1 lead, but Fruhvirtova came roaring back as she reeled off five straight games to seal a fitting end to her breakthrough week.
Rybakina, who in July became Kazakhstan's first ever grand slam champion, was too strong for her Czech opponent, creating 14 break point opportunities in the match while only facing two herself.
Her first serve accuracy was not great at 52 per cent, but she was devastating when she landed it fair, converting 21-of-24 chances.
Rybakina will face Lesia Tsurenko in the next round after the Ukrainian upset fifth seed Ekaterina Alexandrova 7-5 7-6 (7-5).
Second seed Beatriz Haddad Maia also had no issue navigating the challenge of Cristina Bucsa 6-1 6-4, and she will meet Ana Bogdan in the third round after the Romanian defeated Tamara Zidansek 6-1 6-7 (7-5) 6-1.
Meanwhile, in India at the Chennai Open, popular Canadian Eugenie Bouchard collected consecutive wins for the first time March 2021 following a lengthy recovery from shoulder surgery.
The 28-year-old defeated Karman Kaur Thandi 6-2 7-6 (7-2) as she looks to climb back up the rankings, currently sitting 902nd in the world after peaking at number five back in 2014.
Earlier in the day, sixth seed Qiang Wang was upset by Japanese qualifier Nao Hibino 6-2 6-3, while fifth seed Rebecca Peterson was also eliminated, going down 6-4 6-2 to Linda Fruhvirtova.
Wrapping up the day's action, Canada's Rebecca Marino booked a potential showdown with third seed Magda Linette after beating Poland's Katarzyna Kawa 7-5 6-3.
