British number three Kyle Edmund was edged out 6-2 4-6 6-3 by American world number 40 Sam Querrey in the Mexican Open first round.
Edmund, 22, ranked five places lower then Querrey, lost his only other top-level meeting with the 29-year-old at Eastbourne in 2014.
Novak Djokovic will play Juan Martin del Potro in the next round after beating Slovakia's Martin Klizan.
Second seed Rafael Nadal had a 6-4 6-3 win over Germany's Mischa Zverev.