Miami - Canadian top seed Milos Raonic blasted 10 aces in downing US qualifier Tim Smyczek 6-1, 6-4 on Tuesday to reach the second round of the ATP Delray Beach Open.
But Australian sixth seed Bernard Tomic became the first seed ousted at the hard-court event, with Belgian qualifier Steve Darcis rallying to deliver a 3-6, 6-1, 6-4 defeat to the world number 31.
And 37-year-old second seed Ivo Karlovic followed shortly thereafter, falling 7-6 (7/5), 7-6 (7/3) to 69th-ranked American Donald Young despite 19 aces from the world number 20 Croatian.
Raonic, who had beaten Smyczek in the first round of the 2015 US Open in their only prior meeting, booked a second-round matchup with 49th-ranked Croatian Borna Coric, who eliminated Colombia's Santiago Giraldo 6-2, 6-3.
World number four Raonic is seeking a ninth career ATP crown. He has won at least one title in each of the past six years and last year reached his first Grand Slam final, losing to Britain's Andy Murray at Wimbledon.
Darcis, ranked 57th, faced only one break point over the final two sets in fighting back to bounce Tomic in 90 minutes, booking a second-round match against either Germany's Tommy Haas or Georgian Nikoloz Basilashvili, who lost Sunday's ATP Memphis Open final to American Ryan Harrison.
US third seed Jack Sock fired eight aces in downing Moldova's Radu Albot 6-4, 7-6 (7/2). Sock, ranked 21st, next plays Spain's Guillermo Garcia-Lopez, who dispatched Germany's Dustin Brown 6-3, 6-3.
Results on Tuesday from the ATP Delray Beach Open (x denotes seed):
Jack Sock (USA x3) bt Radu Albot (MDA) 6-4, 7-6 (7/2)
Guillermo Garcia-Lopez (ESP) bt Dustin Brown (GER) 6-3, 6-3
Damir Dzumhur (BIH) bt Konstantin Kravchuk (RUS) 6-3, 6-4
Steve Darcis bt Bernard Tomic (AUS x6) 3-6, 6-1, 6-4
Borna Coric (CRO) bt Santiago Giraldo (COL) 6-2, 6-3
Milos Raonic (CAN x1) bt Tim Smyczek (USA) 6-1, 6-4
Steve Johnson (USA x5) bt Stefan Kozlov (USA) 6-1, 6-4
Donald Young (USA) bt Ivo Karlovic (CRO x2) 7-6 (7/5), 7-6 (7/3)
Jared Donaldson (USA) bt Mikhail Kukushkin (KAZ) 7-6 (7/5), 3-6, 6-3
Nikoloz Basilashvili (GEO) bt Tommy Haas (GER) 6-7 (4/7), 7-6 (10/8), 6-2
Juan Martin del Potro (ARG x7) bt Kevin Anderson (RSA) 6-4, 6-4