Laura Robson failed to convert a match point as she lost to Spaniard Lourdes Dominguez Lino in the first round of the Katowice Open in Poland.
Eighth seed Robson held match point on her opponent's serve at 7-6 in the second set tie-break, but could not take it and lost 5-7 7-6 (9-7) 6-1.
Robson replaced Heather Watson as Britain's top woman on Monday after moving up to 39 in the world rankings.
The next event for both players is Britain's Fed Cup tie in Argentina.
That takes place on 20-21 April in Buenos Aires, although Watson's place is not certain as she is currently
Robson, 19, had been hoping to find some form on the clay in Katowice, having won just two of eight matches since the Australian Open in January.
However, 11 double faults and a first-serve percentage of just 46% hampered her chances against the experienced Dominguez Lino.
The 32-year-old Spaniard, a two-time WTA tournament winner on clay, managed to stay in touch before dominating the decider to win in two hours and 30 minutes.