Britain's Katie Boulter lost in straight sets to Slovakia's Anna Karolina Schmiedlova in the first round of the Monterrey Open.
The 24-year-old was beaten 6-4 6-0 in Mexico by the world number 111, who is ranked 207 places higher.
Boulter broke first to go 3-2 up in the first set, only for Schmiedlova to immediately break back and win all but one of the games thereafter.
Heather Watson faces fellow Briton Harriet Dart on Tuesday.
Dart, beaten in the qualifiers, goes into the main draw in Monterrey as a lucky loser after Danka Kovinic pulled out of the tournament injured.
Dart also got into the main draw of last week's Abierto Zapopan event as a lucky loser.
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