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2018: 11th French Open and Monte Carlo titles

Nadal began his 2018 season at the Kooyong Classic, where he lost to Richard Gasquet in the first round. He then played at the Tie Break Tens exhibition tournament in Melbourne, losing in the final to Tomáš Berdych. At the Australian Open, Nadal recorded straight-sets wins in the first three rounds, before notching a tougher four-set win against Diego Schwartzman in the fourth round. He faced Marin Čilić in the quarterfinal, but retired in the fifth set due to a hip injury.

On 16 February, Nadal dropped to the No. 2 ranking after 26 weeks at the top when his rival Roger Federer overtook him in points. Nadal withdrew from the Mexican Open, Indian Wells Open, and Miami Open due to an injury. Despite his absence in Miami, he regained the No. 1 ranking on 2 April due to Federer's second-round loss. After recovering from injury, Nadal helped secure the Spanish Davis Cup team a victory over Germany in the quarterfinal of the World Group. He beat Philipp Kohlschreiber and Alexander Zverev in straight sets.

At the Monte Carlo Masters, Nadal successfully defended his title and won a then-record-breaking 31st Masters title, thus becoming the player with the most Masters titles in tennis history. It also marked his 11th title in Monte Carlo, as well as the 76th title in his career. Because he defended the points won the previous year, he kept his No. 1 ranking and began his 171st week as the world No. 1. Nadal won in Monte Carlo without dropping a set, beating Kei Nishikori in the final. Nadal went on to win his 11th title in Barcelona, defeating Stefanos Tsitsipas in straight sets, becoming the first player in the open era to win 400 matches on both clay and hard. The win marked his 20th ATP 500 series title, which put him back atop the list of most ATP 500 titles, tied with Roger Federer. It also marked his 14th consecutive season with at least one ATP 500 title.

Fresh after achieving the "Undecima" at Monte Carlo and Barcelona, Nadal had to defend yet another title at Madrid. He reached the quarterfinals, defeating Gaël Monfils and Diego Schwartzman in straight sets, to extend his record to 50 consecutive sets won on clay, starting from the 2017 French Open. His win over Schwartzman broke John McEnroe 's record of 49 straight sets won on a single surface. McEnroe had previously achieved the record on carpet in 1984. In a surprise, Nadal lost in straight sets to Dominic Thiem in the quarterfinals, ending his 21-match and record 50-set winning streaks on clay. He also relinquished his world No. 1 ranking to Federer in the process.

At the Italian Open, Nadal captured his 8th title in the Italian capital as well as his 78th career title, defeating Alexander Zverev in three sets, thus overtaking John McEnroe in the fourth place on the list of most titles won in the Open Era. It was Nadal's 32nd Masters title – most of any player in the Open Era. With his victory in Rome, Nadal also regained the No. 1 spot from Federer.

Then at the French Open, Nadal won his 17th Grand Slam title. This tied Margaret Court 's record for singles titles at a Grand Slam event (Court won 11 Australian Opens, but seven came when it was the Australian Championships, an amateur event). En route to the title, Nadal dropped only one set, beating Dominic Thiem in the final in three sets. Nadal became just the fourth man in the Open Era to win three or more major titles after turning 30.

Going into Wimbledon, Nadal was ranked world number one, but was seeded second due to Wimbledon's seeding algorithm. He made it to the quarterfinals without dropping set. He then faced No. 5 seed Juan Martín del Potro, who he defeated in five sets. In the semi-finals he faced long-time rival Novak Djokovic, who was aiming to reach his first major final since the 2016 US Open. This match lasted 5 hours and 17 minutes, spread over two days, becoming the second-longest Wimbledon semi-final in history, second only to the match between Kevin Anderson and John Isner held earlier on the same day. Djokovic defeated Nadal in five sets with the fifth set being 10–8. This was Nadal's first defeat in the semi-finals of a major since the 2009 US Open, and his first-ever defeat in the semi-finals of Wimbledon. Despite this, Nadal achieved his best results at Wimbledon since 2011. This performance, combined with Roger Federer 's unsuccessful title defence, ensured that Nadal retained the world number one ranking after the grass season.

He then won the Rogers Cup, a record-extending 33rd Masters title. This was Nadal's first Masters title win on hard court since 2013. He then withdrew from the Cincinnati Open to prepare for the US Open. Nadal was the top seed during his title defence at the US Open. He first faced David Ferrer in Ferrer's last Grand Slam match, who retired due to injury during the second set. In his semi-final matchup against Juan Martin del Potro, Nadal retired after losing the second set 6–2 due to knee pain. On 31 October, he announced his withdrawal from the Paris Masters due to an abdominal injury and as a result Novak Djokovic replaced him as world No. 1.