Marcos moved into the second round at the US Open on Tuesday, advancing past Mikhail Youzhny when the Russian veteran retired due to cramps in the fourth set.
Marcos led 6-4, 6-4, 2-6, 3-1 when Youzhny was forced to the stop play. The result gives Marcos his first win in New York since 2016, when he registered his best result in reaching the Round of 16.
Youzhny collapsed to the court after the fourth game of the fourth set, clutching his cramping calf muscle. Marcos ran to his aid, assisting his longtime friend.
The win is a significant one for Marcos, who continues to plot his ascent up the Top 100 of the ATP Rankings. Entering the week at No. 93, he is projected to rise at least six spots and will look to continue soaring when he faces 17th seed Lucas Pouille on Thursday.
Four years removed from their lone previous encounter, Marcos will face Pouille with a spot in the third round at stake. They met in 2014 at the ATP Challenger Tour event in Mouilleron le Captif, France.