RIO DE JANEIRO • Rafael Nadal has vowed that he will put in more hard work in a bid to get his tennis game back to the level where he can win more tournaments.

The 29-year-old Spaniard endured another frustrating defeat on Saturday when he lost a marathon semi-final in the ATP Rio de Janeiro Open to 45th-ranked Pablo Cuevas 7-6 (8-6), 6-7 (3-7), 4-6.

It was just the second time since 2005 that Nadal, ranked fifth, has lost to a player ranked outside the world's top 30 on his beloved clay.

That loss came just a week after he lost in the semi-finals of the Argentina Open in three sets to Austria's Dominic Thiem, also on clay.

Nadal had travelled to South America following his first-round loss at the Australian Open in a bid to reverse, on his favourite clay surface, the loss of form that has seen his world ranking slump.

But the latest defeat will cast more doubts on his ability to win the French Open title for a 10th time in early June, having lost to Novak Djokovic in last year's quarter-finals.

"I lost an opportunity, that's it," said Nadal of his loss to Uruguayan Cuevas. "I fought until the end. I have to accept it and keep working to try to change the dynamic.

"I didn't win a title, so it wasn't a positive two tournaments. I had my chances in both. I lost in the semi-finals of both tournaments and just have to look forward to Indian Wells."

That tournament, the first of the year's Masters Series events, gets under way in California on March 20, followed by the Miami Masters.

Both tournaments are on outdoor hard courts and Nadal will be hard pushed to match world No. 1 Djokovic, who has already won the Australian Open in dominating fashion.

In Florida, Juan Martin del Potro's comeback from a near-12-month injury layoff ended in defeat on Saturday, as the 2009 US Open champion fell to Sam Querrey 5-7, 5-7 in the ATP Delray Beach Open semi-finals.

Querrey fired 10 aces and won 30 of 34 points on his first serve to advance after 80 minutes without ever facing a break point, booking a date in the final against fellow American Rajeev Ram, who upset Bulgarian fourth seed Grigor Dimitrov 6-4, 6-3.

Del Potro, ranked 1,042nd in the world, was playing in his first event since last March following left wrist surgery last June.

He showed flashes of his old form, displaying good court coverage and growing confidence in his ground-strokes throughout the week, save for two poor service games against Querrey.

"Sam was really smart playing against me," said del Potro. "He served really well... I couldn't move fast on court and I missed too many forehands and he played at a higher level than me."

The 27-year-old wildcard entrant, a former world No. 4 who stunned Roger Federer in the 2009 final at Flushing Meadows, missed most of the 2014 and 2015 seasons owing to the wrist injury.

"I always try to win, that's for sure," he said. "But this week I won more than a tournament. I'm so glad to play tennis again."

Querrey, ranked 61st, had lost both previous meetings with the South American star without taking a set but had not faced him in nearly five years.

AGENCE FRANCE-PRESSE, REUTERS