Former champion Juan Martin del Potro admitted on Monday he was shocked to be back in the quarter-finals of a Grand Slam for the first time in more than three years.
Del Potro became the lowest ranked player in 25 years to reach the US Open last-eight when Dominic Thiem quit their last-16 tie with a knee injury.
Del Potro, the champion in New York in 2009 but whose ranking at the start of the US Open was down at a lowly 142, last got this far at a major in 2013 when he was a Wimbledon semi-finalist.
“I didn't expect to be in the quarters in my second Grand Slam after my comeback,” said the giant Argentine, whose appearance at Wimbledon this summer was his first at a major since the Australian Open in 2014.
“I'm playing good tennis. My level is going up every day. I'm so glad to be part of the last eight and that I will have the chance to play another big challenge. Anything can happen.”
Del Potro was leading 6-3, 3-2 when Thiem, the eighth-seeded Austrian, retired with a right knee injury having received a medical timeout just moments earlier.
Del Potro, who was pushed to the brink of retirement earlier this year after undergoing three surgeries on his left wrist, will now face third seed Stan Wawrinka.
He has a 4-2 winning record over the 31-year-old Swiss, including a win at Wimbledon this year which gave him four in a row.
Wawrinka hasn't beaten del Potro since 2008 but the two have never met on hard court.
Win or lose on Wednesday, del Potro said he is getting stronger with every match he plays as he rebuilds his career after being on the brink of retirement this year.
“I was close to retiring, but after that (the surgery) I always believed in myself to come back on tennis. When I made the decision to do the surgery, I always believed I will come back,” said the 27-year-old.
“Now I'm here. I'm playing free. I don't have any big problems in my wrist, and that's important. Sometimes it bothers me a little bit, but I can deal with that. I'm still improving my backhand.
“Now I have to hit my backhand even better than today for my next match. Here is a different surface than Wimbledon, so my slices don't go too well. But I'm OK with my level."
Jimmy Connors was ranked 174 when he went to the US Open semifinals in 1991 at the age of 39.
Should del Potro go on and get to the final, he would be the lowest-ranked man in the modern era to make a Slam championship match.
