Roger Federer in straight-set victory over Philipp Kohlschreiber

Five-times U.S. Open winner Roger Federer took another straight-sets step at the 2015 championships with a 6-3 6-4 6-4 third-round victory over Philipp Kohlschreiber at Flushing Meadows on Saturday.

Federer lost his serve for the first time in the tournament as Kohlschreiber broke him twice in the 93-minute match, but the 34-year-old Swiss stayed in command with five service breaks of his own to improve his career record to 10-0 against the German.

"I think I won the big points today," said Federer, who cashed in five of eight break point chances. "I served well when I had to at the end."

Next up for Federer in the Round of 16 is big-serving John Isner, who advanced when Czech Jiri Vesely retired due to a neck injury when trailing 6-3 6-4. The 13th-seeded American hammered 21 aces and 46 winners past Vesely in the abbreviated match.

The evergreen Federer, the grand slam king with 17 titles but none since the 2012 Wimbledon, has been brutally efficient with his own serve during a strong season and an impressive build up to the U.S. Open. He was a finalist at Wimbledon in July and has captured five ATP titles this year, including last month's Cincinnati Masters, where he defeated world number one Novak Djokovic in the final.

Federer dropped serve for the first time since Wimbledon, when Kohlschreiber broke in the fourth game of the second set. Federer had held in 78 consecutive service games in Cincinnati and the U.S. Open combined.

The second seed said Isner presented a new challenge. "He's got the power. He's so tall, he finds the impossible angles for us," Federer said of the American's serve. "And has a great second serve as well. "I need to make sure I protect my own serve. Part of trying to beat him is just to stay with him." For Federer, service success includes a cerebral component. "It's focusing point by point," he said.

"Understand the percentages, what has and hasn't worked so well throughout the match.

"In the beginning you try to find the rhythm. Then once you found it, how much do you mix up speeds and slices and big serves to keep him off balance.

"I tend to like the body serve as well," added the Swiss tactician. "Sometimes against John maybe that's not a bad play just because he's very long."