Marcos needed one hour and 43 minutes to upset second seed Vasek Pospisil 6-4, 7-6(5) in Friday's BB&T Atlanta Open quarter-finals.
Marcos stayed the course after failing to convert on six set points in the first set, before eventually closing it out 6-4. In the second set, he rallied from a 1-4 tie-break deficit, reeling off six of the next seven points to close out the match. A jaw-dropping running forehand winner set up his first match point, which he would claim.
"I think I was controlling the match until 5-2 when I served for the first set at 40/15," Marcos said. "I lost concentration there and gave some free points. I think he was playing more aggressive than me.
"In the end I found a way to win. I turned it around and played an unbelievable point at 5-all in the tie-break. I won the most important points and that made the difference."
Marcos will contest his third ATP World Tour semi-final of the year (Zagreb & Nottingham), facing old friend and seventh seed Gilles Muller. It will be their first meeting as professionals, having competed against each other twice in their junior days.
"This match was very important for me," Marcos added. "I'm coming back from injuries and it's a huge relief that I can beat these guys who are serving and playing so well. I don't think I played a very good match. I was really excited and emotionally it was an unbelievable feeling."
