Watson dropped 10 places in the latest world rankings
British teenager Heather Watson was beaten 7-6 (7-4) 6-3 by world number 33 Elena Vesnina in the opening round of the Family Circle Cup in Charleston.
The 17-year-old US Open junior champion beat Ekaterina Dzehalevich and Zarina Diyas to reach a second WTA Tour event.
Both players had breaks in the opening set before Vesnina won the tie-break.
Watson took the first break of the next set but Vesnina responded immediately and then broke twice more to complete victory in an hour and 47 minutes.
Russian Vesnina is seeded 10th for the tournament in South Carolina, which has a prize fund of $700,000 [£455,600].
On Monday, Watson's world singles ranking slipped slightly, following her career high last month of 406 in the world, to 416.
However, she moved up one place in the doubles rankings to 452 in the world, which is a career high.
Meanwhile, British number one Elena Baltacha moved through to the second round of the ITF event in Johannesburg with a 6-2 6-2 win over Vitalia Diatchenko of Russia.
Baltacha will now play Katherin Woerle on Wednesday and said of her victory: "I'm really happy with the way I played. First round matches are never easy and I felt like I dealt with the heat and the attitude pretty well. I'm looking forward to playing Woerle."