Nadal, who is the clear tournament favourite and looks to be in the form of his life on clay this season, raced into a 3-0 lead before Spanish compatriot Almagro felt his left knee with less than half an hour played.
One of sport's great rivalries, Roger Federer and Rafael Nadal have clashed 37 times over the years but never stood across from each other on Flushing Meadows' hardcourts.
Images from Day 5 of the US Open in New York on Friday
Teenager Taylor Fritz earned the chance to face his hero Roger Federer after the American defeated an unranked French qualifier on Tuesday to reach the second round in Stuttgart. Federer will be playing his first match in nearly a month after recovering from back pain and skipping the French Open, as he ramps up his preparations for Wimbledon. Number 65 Fritz has had time on court with the Swiss legend -- a Wimbledon training session a year ago - but booked his place into the match of his life with a 3-6 7-6 (8/6) 6-1 win over Fabrice Martin on the grass at the Weissenhof Club. Fritz, 18, said he will not worry with nerves when he faces one of the best players on the planet. "I have nothing to lose, there is no pressure on me, I don't think there will be any nerves." The teenager said he had watched and admired Federer as a boy aged 11.
Swiss great Roger Federer insists he will end his career this week with no regrets even though his record of 20 Grand Slam titles was overhauled by Rafael Nadal and Novak Djokovic.
Images from Day 3 of the French Open matches played at Roland Garros in Paris on Tuesday
World number two Novak Djokovic hammered Swiss Stanislas Wawrinka 6-1, 6-4 to reach the Paris Masters semi-finals on Friday.
Top seed Roger Federer was finally forced to work hard for a win at the Swiss Indoors tournament as he fought back to beat Adrian Mannarino 4-6, 6-1, 6-3 in his quarter-final on Friday.
Top seed and defending champion Roger Federer battled past in-form Russian Daniil Medvedev 6-4, 4-6, 6-4 to reach the last-16 at the Shanghai Masters on Wednesday.
Former world number one Garbine Muguruza was beaten 7-5, 6-4 by Aryna Sabalenka in the China Open second round on Tuesday while an ailing Jelena Ostapenko lost 6-0, 6-0 to Wang Qiang.
Six-time champion Novak Djokovic's hopes of turning around his season at the Miami Open ended in the second round when he was defeated 6-3, 6-4 by Frenchman Benoit Paire.
Despite the world number nine's withdrawal, a strong field is expected for the event, including Roger Federer, Kei Nishikori, Milos Raonic, Grigor Dimitrov and Juan Martin del Potro, who have all confirmed their participation.
IMAGES from the matches played at the PNB Paribas Open at Indian Wells on Wednesday
Images from matches played on Day 4 of the Australian Open at Melbourne Park on Thursday
Cilic's booming serve lacked its usual accuracy and Germany's Kohlschreiber was able to control long rallies, pushing the towering Croatian around the court with pinpoint groundstrokes en route to a quick victory.
Canadian teenager Denis Shapovalov believes his first season on the ATP Tour has helped him grow into a better player and he is relishing the challenges ahead.
Former world number three Nikolay Davydenko announced his retirement on Thursday at the age of 33.
Nadal, who got a first-round bye, defeated Richard Gasquet 6-3, 6-4 to advance.
Serena Williams returns to the Arthur Ashe Stadium on Sunday
Marin Cilic overcame Federico Delbonis 6-3, 7-5, 3-6, 1-6, 6-2 after a titanic tussle in the opening singles rubber to give hosts Croatia a 1-0 lead over Argentina in the Davis Cup final on Friday.
Since his June trek with his wife up Mont Sainte-Victoire, the mountain that famously also served as an inspiration for Paul Cezanne, the 31-year-old has added a pair of Grand Slam titles to his resume.
India's No 1 singles player Somdev Devvarman on Monday jumped 18 places to become World No 78 in the latest ATP rankings. The tennis star had his Delhi Open triumph to thank.
Serena Williams faces her first seeded opponent at the French Open on Thursday and will hope to use her booming serve to beat Australian Ashleigh Barty in the second round.
Former world number one Serena Williams returns to Grand Slam action for the first time in over 15 months, when she goes up against Czech Kristyna Pliskova in the French Open first round.
Rafael Nadal continued to scythe his way through the Shanghai Masters on Thursday, reaching the quarter-finals with a 6-3, 6-1 thrashing of temperamental Italian Fabio Fognini.
Lucky loser Taro Daniel will replace Nadal in the second round against Jo-Wilfried Tsonga.
It was Kristyna's first win over a top-five ranked player and she will go on to play fellow Czech Barbora Strycova in the quarter-finals.
World number two Novak Djokovic expects Stanislas Wawrinka and Andy Murray to lift their game at the French Open despite failing to make an impact so far in the clay court season.
Less than a month after her run to the Wimbledon final, where she lost to Spain's Garbine Muguruza, 37-year-old Williams again had to turn back time against a young challenger.
Here is a timeline of Djokovic's decline since the last of his 12 Grand Slam titles.
The seventh-seeded Bulgarian advanced to his eighth ATP World Tour Masters 1000 quarter-final with a 6-3, 7-5 victory over Del Potro, who struggled with the intense heat in the closing stages of the one hour, 39 minute match.
Sania Mirza and her Czech partner Barbora Strycova stormed into the quarter-finals of the BNP Paribas Open 2017 tennis tournament with a straight-set win over their opponents in the women's doubles at Indian Wells.
His great rival Roger Federer had already maintained his regal progress earlier on the showcourt and Serena Williams then followed with another thumping win.
Raucous home support for holders Argentina did not make up for the absence of Juan Martin del Potro and Federico Delbonis on Friday as they fell 2-0 behind to Italy in the first round of the Davis Cup.
Images from Day 3 of the French Open in Paris on Tuesday
Roger Federer took a big step towards securing his endangered place in the ATP World Tour Finals by overcoming rising Canadian talent Vasek Pospisil 6-3, 6-7(3), 7-5 in the Basel Open semi-finals on Saturday.
World number one Simona Halep saved three match-points before edging past an inspired Lauren Davis 4-6, 6-4, 15-13 in a thrilling marathon to reach the fourth round of the Australian Open on Saturday.
IMAGES from matches played on Day 7 of the French Open at the Roland Garros in Paris on Saturday
Alexander Zverev became the first German to reach the last four at the ATP Finals for 15 years as he beat American John Isner 7-6(5), 6-3 at the O2 Arena on Friday.
Rafael Nadal gave Russian teenager Andrey Rublev a tennis lesson on Wednesday, racing into the semi-finals of the US Open with a ruthless 6-1, 6-2, 6-2 win that cleared the path for a possible showdown with rival Roger Federer.