Britain's Harriet Dart beat compatriot Heather Watson in straight sets in the first round of the Monterrey Open.
Dart - who is third in the British rankings, one place lower than Watson - won 6-4 7-6 (7-1) in one hour 39 minutes in Mexico.
She was beaten in qualifying but made the main draw as a lucky loser after Danka Kovinic pulled out through inury, while Watson, 28, was seeded fifth.
World number 153 Dart, 24, will play Viktoria Kuzmova in round two.
The Slovakian world number 104 beat American wildcard Caroline Dolehide 6-3 6-1 in round one.