Former US Open champ Juan Martin Del Potro got his season off to another successful start at the Delray Beach Open, defeating Kevin Anderson in straight sets and overcoming an opening set tumble in the process.

Del Potro wasn't at his best, especially on his backhand. But he used his powerful serve and blasted enough forehand winners to beat the South African 6-4, 6-4.

Del Potro, who also started his ATP Tour season last year at the Delray Beach event, hammered five aces and won 84 percent of his first serve points in the first round battle between two former tournament winners.

In another marquee matchup Tuesday, Canadian top seed Milos Raonic blasted 10 aces in downing US qualifier Tim Smyczek 6-1, 6-4 to reach the second round.

Del Potro reached the semi-finals of this event last year before going on to win 32 matches and being named the Tour's comeback player of the year for the second time.

The 28-year-old Argentinian, who won the 2009 US Open title, has endured three left wrist and one right wrist surgeries in his career.

On Tuesday, Del Potro looked tentative at times but he was also able to generate enough pace on the ball to thwart Anderson's serve and volley attack.

He fought through four double faults and a grazed right knee he suffered in a fall in the first set.

Del Potro won the Delray Beach title in 2011 and Anderson took the 2012 crown.

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.

'Best shape'

"Overall I have to be happy," Raonic said. "I haven't played much tennis over the last few weeks, and I really put in as much work as I could over the last week to get into my best shape here, and now I'm here.

"I've won my first match and I hope I can continue. I feel good."

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 streaking 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.

Both Del Potro and Haas both launched comeback bids Tuesday with Haas losing 6-7 (4/7), 7-6 (10/8), 6-2 to the red-hot Basilashvili.

Haas underwent right foot surgery last April and had not played since October 2015 until retiring from a first-round Australian Open match last month.