Fifth seed Alexandr Dolgopolov slumped to a first-round loss to Croatian Ivan Dodig at the Delray Beach International Tennis Championships.
The 22nd-ranked Ukrainian continued his underwhelming opening to the 2013 season, with a 6-3 6-3 defeat to the unseeded Dodig in 57 minutes at the US hardcourt tournament.
Dolgopolov's match record on the ATP Tour slumped to 3-4 in the calendar year, after the defeat at the ATP 250 event.
Dodig, meanwhile, took his record for the season to a positive 5-4 on the Tour, which has included a third-round Australian Open appearance and a quarter-final exit in Zagreb.
He will meet another player who reached the last 32 at Melbourne Park in January, Russian Evgeny Donskoy, who dismissed Belgium's Steve Darcis 7-6 (7-5) 6-3.
Donskoy won 27 of 32 points when he landed his first serve, and he converted three of five break-point opportunities.
Eighth seed Xavier Malisse enjoyed better fortune than his countryman Darcis, as he dismissed Colombian Alejandro Falla 6-3 6-3.
Malisse was forced to face 10 break points, but he saved nine of them, while converting four of the five chances he had on left-hander Falla's serve.
He will meet either American Ryan Harrison or Spanish qualifier Daniel Munoz-De La Nava next.
In the final match of the day, third seed Sam Querrey won an all-American battle over tour veteran Michael Russell 6-2 2-2 when the latter retired with a leg injury.