Novak Djokovic of Serbia (left) reacts after losing to Argentina's Juan Martin Del Potro on Sunday.?[Photo/Agencies]??

Novak Djokovic shook his head and placed his palm over welling eyes on his way to the locker room after a first-round Olympic exit.

The world No 1 was devastated he might never realize his goal of adding Olympic singles gold to a substantial collection of Grand Slam titles.

"I mean, no doubt, it's one of the toughest losses in my life, maybe my toughest ever," Djokovic said, shaking his head and speaking in subdued tones. "Not easy to handle."

The Serb was hardly alone in his disappointment on Day 2 of the Games. He was one of three top seeds to exit the tennis competition in a wild span of 12 hours.

Djokovic's 7-6(4), 7-6 (2) setback on Sunday night was caused in large part by ferocious forehand after ferocious forehand from a resurgent Juan Martin del Potro of Argentina.

The surprising result followed losses by the top-seeded duos of Serena and Venus Williams of the US - who entered their match with a 15-0 Olympic record and three gold medals as a pair - in women's doubles, and Nicolas Mahut and Pierre-Hugues Herbert of France in men's doubles.

The second-seeded men's doubles duo of Andy and Jamie Murray departed, too.

"Got to take it on the chin and move on," said Jamie, the older brother of two-time Wimbledon winner Andy.

Sunday began with 25 mph winds that delayed the start of play on eight of Rio's nine courts for about two hours, and then came so many startling outcomes.

None was more significant than 2009 US Open champ del Potro's victory over 12-time major champion Djokovic in a thriller before a raucous crowd.

The match concluded with both men, who are friends, wiping away tears after a lengthy, warm embrace at the net.

Del Potro has dealt with three operations to his left wrist that kept him off the Grand Slam circuit for 2?years, until Wimbledon last month, and even he acknowledged: "I didn't expect to beat Novak tonight."

Djokovic had won seven of their eight most recent meetings, and 11 of 14 overall, before Sunday.

This is the second time del Potro came out ahead at an Olympics: He beat Djokovic to take the bronze medal in London four years ago.