Miami, FL (SportsNetwork.com) - Former top-five star Juan Martin del Potro came up a first-round loser at the Miami Open on Thursday in his first match since mid-January.

Canada's Vasek Pospisil dismissed his fellow non-seed del Potro 6-4, 7-6 (9-7) on the hardcourts at Crandon Park.

"I don't feel frustrated," del Potro said. "I have to take the positive things on my comeback. The score doesn't matter for now.

"I didn't serve well in the special moments," the Argentine added. "But now I'm OK. Just playing the tournament and being here means a lot for me."

The oft-injured del Potro had been sidelined for more than two months with a left wrist injury. He finished last season outside the top 100 for the first time since 2005 after appearing in just four tournaments. The 2009 U.S. Open champion opened 2014 with an 18th career title in Sydney, but underwent surgery in March and missed the rest of the season.

In Thursday's finale, Thomaz Bellucci upended former world No. 1 Lleyton Hewitt 7-5, 6-7 (2-7), 6-4 in a match that lasted nearly three hours.

Earlier Thursday, two-time NCAA singles champion Steve Johnson was beaten by Mikhail Kukushkin 6-3, 3-6, 6-2, and Simone Bolelli subdued fading former Australian Open runner-up Marcos Baghdatis 6-2, 6-2.

Additional opening-round wins came for Alexandr Dolgopolov, Jerzy Janowicz, 18-year-old Croatian Borna Coric, Alejandro Falla, Mikhail Youzhny, Teymuraz Gabashvili, Alexander Zverev, Steve Darcis, Federico Delbonis, Andrey Rublev and Jurgen Melzer, who beat American Ryan Harrison in straight sets.

No. 1 seed Novak Djokovic is the defending champion and will be seeking his fifth Miami title. Last week's Indian Wells titlist beat four-time Miami runner-up Rafael Nadal in last year's marquee final in South Florida, which was a rematch of the 2011 final also captured by Djokovic.

Djokovic will open Saturday against Martin Klizan, who beat Paolo Lorenzi 6-3, 6-4 on Day 2.

The winner of this $5.4 million event will claim $900,400.