Spain's Rafael Nadal, the world's top-ranked tennis player, has said that despite a torrid start to his clay-court campaign it was unlikely he would be able to overcome an injury hiatus earlier in the season and finish as tennis' year-end No.1 for the fifth time.

Nadal, in an interview with Efe here on Thursday, made the remarks ahead of the start of next week's Madrid Open, where he will try to win a third straight clay court tournament in 2018.

"I've given away (due to injuries) three months of the year," Nadal said, referring to his time away from the ATP World Tour from late January, when he retired in the fifth set of his quarter-final match against Croatia's Marin Cilic at the Australian Open due to a hip injury, until the start of the Monte-Carlo Masters in mid-April.

"Not playing for three months and finishing No.1 in the world, I'd say it's not impossible but it's very unlikely," the 16-time Grand Slam champion said at a Kia Motors event, reports Efe.

Nadal has a slim lead in the ATP rankings over arch-rival and world No.2, Roger Federer, although the 36-year-old Swiss great has opted to reduce wear-and-tear on his body by skipping the entire clay court season for the second straight year.

The 31-year-old Spaniard did not drop a set in winning two Davis Cup matches in early April and capturing titles at the Monte-Carlo Masters and the Barcelona Open -- each for the 11th time -- and now has an Open Era-record streak of 46 consecutive sets won on clay dating back to last year's French Open.

"I'm happy about how things are going, especially after a difficult start to the season with injuries. Both in Davis Cup and in Monte-Carlo and Barcelona, things turned out as well as possible," the Kia global ambassador said.

Although he acknowledged that his biggest short-term goal was to increase his record haul of French Open titles to 11, he said the Madrid Open -- a key tune-up for Roland Garros, which starts later this month -- was "one of the year's most important tournaments."

"Playing at home is special. My relationship with the crowd in Spain is very special. They totally support you and I'm very thankful for that," the Iberian nation's greatest-ever tennis star said.

Nadal also confirmed that health permitting he would be available for Spain's away Davis Cup semi-final tie in September against France, saying he had given his word to captain Sergi Bruguera.