Tennis-Del Potro into U.S. Open quarters after Thiem retires

* Del Potro still has three hours rehab a day * Blisters cause Thiem knee problems (Adds quotes, detail) By Steve Keating NEW YORK, Sept 5 (Reuters) - Juan Martin del Potro's summer renaissance continued on Monday as the unseeded Argentine advanced into the U.S. Open quarter-finals when Austrian eighth seed Dominic Thiem quit their match with an injury. Del Potro, the 2009 U.S. Open winner, whose career has been interrupted by a series of wrist operations and needed a wild card to get into the year's last grand slam, has made the most of his opportunity by reaching the last eight without dropping a set. The towering Argentine's path was made a little easier when Thiem retired with a knee injury trailing 6-3 3-2. "I had some problems with blisters and maybe because of the compensation the knee started to hurt," explained Thiem. "I'm going to do an MRI now in the next couple hours, and then I will have more information about it. "At the beginning of the match it was basically ok but...it got worse and worse. No other choice." Just 15 minutes into the match and Thiem up a break 2-1, it looked like Del Potro might be the player in danger of making an early exit when he called on trainers to work on his right shoulder during the changeover. The problem, however, was not a major one, the 27-year-old Argentine returning to court and eventually breaking back at 3-3, sweeping four straight games and closing out the set with a thundering ace. Del Potro continued to pressure the young Austrian in the second taking an early break to go up 2-1, Thiem letting out a yelp of pain chasing down a return. Thiem attempted to continue but after a brief discussion with trainers called it a day, shaking Del Potro's hand and exiting the court. After undergoing three wrist operations, which still require around three hours of rehab every day, Del Potro continues to surprise himself reaching the last eight in Flushing Meadows. Just before his third surgery Del Potro admitted he considered retirement but six months into his latest return to competition and a Rio Olympic silver medal in the bag, the Argentine is penning one of the great tennis comeback stories of all-time. "I was close to retire before my third surgery but after that I always believe in myself to come back," said Del Potro, who will face the winner between third seed Swiss Stan Wawrinka and Ukraine's Illya Marchenko. "When I made the decision to do the surgery, I always believed I will come back, for sure. "I didn't expect to be in quarters in my second grand slam after my comeback. "My level is growing up every day. "Everything can happen." (Editing by Frank Pingue/Rex Gowar)