British number one Heather Watson suffered a first-round defeat by Croatia's Donna Vekic at the clay-court Family Circle Cup in Charleston, USA.
Watson, seeded 16th and ranked 67 places above Vekic at 41st, lost 6-3 4-6 7-5 in two hours and 36 minutes.
The Briton twice led by a break in the final set and had looked on course for victory serving at 4-3, 40-0.
It was only a third win of 2015 for 18-year-old Vekic, who has slipped outside the top 100 from a high of 62 in 2013.
Watson, 22, went into the match with 10 wins and a WTA title to her name this year but dropped serve immediately and could not recover in the opening set.
The Briton's situation was becoming dire by the time Vekic moved 4-2 clear in the second, but the Croat crumbled with three double faults in a single game as she was broken, and then failed to capitalise from 0-40 on the Watson serve.
|Vekic finds her form|
|The 18-year-old, who split with British coach David Felgate last year, became the youngest winner of a WTA title since 2006 when she won in Kuala Lumpur aged 17 years, 10 months in 2014.|
|In 2015 she has lost in the first round at Shenzhen, the Australian Open and Pattaya City, and in qualifying at Sydney and Miami.|
Watson took advantage to complete a four-game sweep and level at one set all, and then dragged Vekic out of position to break for 2-1 in the decider.
There were more twists to come, however, with Vekic calling for the trainer three times as she suffered with cramp in her foot, only to recover two breaks - the second from 40-0 down - with some fearless hitting.
Watson was faced with serving to force a decisive tie-break but Vekic kept up the attack and broke to love, clinching victory with a backhand winner.