By Steve Keating NEW YORK, Sep 3 (Reuters) After two marathons, Roger Federer had the after-burners on, sprinting past Spaniard Feliciano Lopez 6-3 6-3 7-5 and into the fourth round of the U.S. Open on Saturday. Chasing a sixth U.S. Open title, Federer needed just one hour and 46 minutes under a closed roof at Arthur Ashe Stadium to put away the 31st seed Lopez, leaving plenty in the tank after being forced to go the distance in his opening two matches. It has been a far different start to the U.S. Open than Wimbledon where an economical Federer stormed to his 19th grand slam title without dropping a set. In his opening two matches at Flushing Meadows, a misfiring Federer had been forced to five sets, first by American young gun Frances Tiafoe then Russian veteran Mikhail Youzhny. After being broken eight times and committing a whopping 124 unforced errors in those first two contests, a focused Federer tightened up his game against Lopez, operating with his hallmark efficiency. "I struggled the first couple of rounds but I think that had something to do with my opponents. They came out and played very well," Federer said in his post-match interview on court. "Once I got off to a slow start and once my opponent fought back really hard and today it almost happened again in the third set but I think I played a very clean match. "I couldn't be happier to get a match in straight sets against Feliciano who has been a great competitor over the years." A capacity crowd saw Federer try out everything in his tool bag. The timely big serve, the elegant backhand, laser forehands, hustle and defense were all on display. He committed just 16 unforced errors while firing 32 winners to run his record to 13-0 against the 35-year-old Spaniard. Federer started the match as if he had dinner reservations, breaking Lopez to go up 4-2 then holding serve to clinch the opening set in a snappy 25 minutes. He marched on, his focus only faltering slightly in the third set when he allowed Lopez to break back. But the 36-year-old Swiss just buckled down and broke Lopez again to close out the match and set up a fourth round meeting with German Philipp Kohlschreiber. REUTERS SV 1011