John Isner fell at the first round in the Mexico Open after fellow American Ryan Harrison picked up a 6-3 7-6 (7-5) win.
Big-serving Isner, the eighth seed in Acapulco, failed to find his rhythm in the first set and was edged out in a tie-break in the second, with Harrison progressing after saving seven of eight break points in total.
Fifth seed Kevin Anderson, the 2014 runner-up, made a safe passage with a 6-4 6-3 win over Radu Albot of Moldova while David Ferrer saw off Russia's Andrey Rublev 6-4 6-3.
American Jared Donaldson also made it through to the second round in Mexico after Nikoloz Basilashvili retired hurt when trailing 6-2 3-2.
Spain's David Ferrer advanced in straight sets, defeating Russia's Andrey Rublev 6-4 6-3.
Ferrer dominated the match, which lasted one hour and 23 minutes, paying the 20-year-old Rublev back for beating him last month in the first round of the Australian Open.
"I want to rest a little and be ready to prepare (for) each game," Ferrer said after the match, in which he won 65 per cent of the service points compared to his opponent's 55 per cent.
The 35-year-old is set to play the winner of Tuesday's duel between Juan Martin del Potro of Argentina and Mischa Zverev of Germany.
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