The 2009 US Open champion was playing his first match since a January loss to Marin Cilic in the fourth round of the Australian Open. He has since been sidelined with a wrist injury that required surgery.
"I played a good match," Del Potro said. "I had my chances in the first-set tiebreak and he played some great points. I'm just happy to be back on the circuit and I hope I get better and better. He played very well today."
Story continues below ↓advertisement | your ad hereRochus took 1 hour, 44 minutes to knock out the former world No. 4.
Top-seeded Rafael Nadal will play his first match since winning the US Open and completing a career Grand Slam when he faces Belgian qualifier Ruben Bemelmans on Thursday.
Bemelmans won the first ATP match of his career on Tuesday when he beat fellow qualifier Federikson Nielsen of Denmark.
"Winning my first ATP match is a great thing, and to get to play against Rafael Nadal in the second round is just an extra," said Bemelmans. "I will just go on court, enjoy the match and see what happens.
"It's not going to be easy against him, he's the No.1 player and already a legend, and he's only 24. I'll give everything I have on court and let's see what it will bring."
Rochus, who stands 1.68 meters (5-foot-6) tall to Del Potro's 1.98 (6-foot-6), was solid from the baseline and relied on strong counterattacking against an erratic opponent.
Del Potro faced set point at 6-5, 30-40, but he escaped by blasting three successive aces to force the tiebreak. The Argentine then fought back from 0-3 down to earn a set point of his own following a backhand winner, but Rochus replied with his own sharp backhand to level at 6-6 before a powerful forehand wrapped up the first set in 55 minutes.
Del Potro surged into a 2-0 lead in the second set but unforced errors cost him the next three games to fall down a break at 2-3. At 3-5, Del Potro took risks with big shots to save two match points, but his resistance did not last long and Rochus sealed victory in the next game when the Argentine's backhand sailed wide.
In other first round play, eighth- seeded Michael Berrer of Germany also suffered a first round exit, when Guillermo Garcia-Lopez of Spain stunned him 6-4, 7-6(4) in 1 hour, 33 minutes.
Germany's Rainer Schuettler beat Ricardo Mello of Brazil 6-4, 6-2, Dudi Sela of Israel defeated Konstantin Kravchuck of Russia 6-3, 6-4, and Mikhail Kukushkin of Kazakhstan battled to a 6-3, 6-7 (4), 6-4 victory over Ryler DeHeart of the U.S.
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