After slamming his water bottle to the clay in frustration, Almagro returned to his chair - and was joined by del Potro, who hung around and offered words of encouragement to the new dad, whose son was born earlier this year.
Del Potro walked with a weeping Almagro towards the umpire's chair and after the Spaniard officially retired, the two shared a hug. Of course, it's not easy for me when you have a friend on the other side of the court showing an injury or crying.
Former US Open champion Del Potro is no stranger to injury, with the 28-year-old appearing at Roland Garros for the first since 2012 after being pushed to the brink of retirement.
At 1-all in the third set - del Potro took the first, then lost the second - there was the startling way Almagro paused at the baseline, placed his hands on his knees and shook his head, then fell backward onto the court.
World number 50 Klizan has already hit the headlines in Paris by being accused of faking an injury in his opener against French wildcard Laurent Lokoli.
However, the Scot insisted he is playing well enough to beat Del Potro after battling past Martin Klizan.
The Swiss, 32, won 6-4 7-6 (7-5) 7-5 against the world number 89.
'After tomorrow it could be another great battle if I feel good. "I tried to play a solid match, I hit deeper the longer the match went on and tried to dictate the points".
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Following the withdrawal, del Potro was given a free pass into the third round.
Del Potro didn't have to finish the match, as Almagro was forced to retire due to a knee injury, saving the Argentinian extra tennis considering the match was going at least four sets. Yes, it's something I constantly try to get better at.
Covering his face, the sobbing 31-year-old bent over behind the baseline before collapsing onto his back.
Del Potro will advance to play Andy Murray.
"I just do what my heart feels, you know?"
Murray produced an encouraging four-set win over Andrey Kuznetsov in round one, but the early signs were not good against the unsafe Slovak.
World No. 3 Stan Wawrinka came out on top in his French Open slugfest with Ukraine's Alexandr Dolgopolov, winning 6-4 7-6 (5) 7-5 on Thursday to set up a third round showdown with Italian maverick Fabio Fognini.
Controversial Australian 18th seed Nick Kyrgios is also out after he was beaten 5-7 6-4 6-1 6-2 by South African Kevin Anderson.