WHAT HAPPENED: For the second straight match, it was vintage Roger Federer, as the 19-time Grand Slam champion let everyone know he isn't going anywhere. The five-time US Open winner defeated Philipp Kohlschreiber in straight sets in Arthur Ashe Stadium on Monday to advance to the quarterfinals.

The match ended in one hour, 49 minutes, on the heels of Federer's third-round win in one hour, 46 minutes against Feliciano Lopez on Saturday. Each of Federer's first two matches in Flushing prior were five-setters, but the 36-year-old put any questions of fatigue to rest on Monday.

Federer only dropped six points on his five service games, breaking once to take the first set 6-4. In the second set, Federer broke Kohlschreiber twice, riding his serve to a 6-2 set win, losing just four points in four service games.

The third set was more of the same, with Federer not giving Kohlschreiber any opportunities. The two each held serve until 5-5, when Federer broke. He then closed out the match on his serve. In all, Federer dropped 13 points on his 15 service games and didn't face a break point all match.

"I didn't know that, but I felt like I didn't drop my serve and that was a first in this tournament and that was a great feeling because one break wins you a set," Federer said on court after the match.

play video Federer vs. Kohlschreiber

Federer has now defeated Kohlschreiber all 12 times they've faced, including in the third round of the 2015 US Open. He's dropped three of 30 sets in their head-to-head meetings.

Also, Federer has claimed 18 straight Grand Slam matches, having won the Australian Open and Wimbledon this year and skipping the French Open in order to prepare for the grass- and hard-court season. He's one win away from reaching the semifinals in his seventh straight Slam, which included making the 2015 Wimbledon and US Open finals. Federer missed the French Open and US Open last year because of a knee injury.

Kohlschreiber, who entered the match having not dropped a set in the tournament, now has lost his last 29 tour matches against opponents ranked in the Top 10. He's also 1-9 in the fourth round of majors in his career.

Next up for Federer is Juan Martin del Potro, who ended Federer's run of five straight US Open titles by beating him in five sets in the 2009 championship match. Del Potro defeated Dominic Thiem Monday after trailing two sets to love.

"It's great to see him back on the tour. It' s been a rough ride for a lot of years with his wrists," Federer said. "To play him again this year after our epic final in 2009 ... I am looking forward to it.

"Probably feels like one of those matches I would like to play over again. Feel like I would probably win it somehow because I should have been up maybe two sets to love ... I just had all these chances in multiple moments. The only time when he was really better, in my opinion, was the fifth set. Obviously that was good enough to beat me that day.

If Federer wins that match, he'd play Rafael Nadal if the Spaniard beats Russian Andrey Rublev in the semifinals.

WHAT IT MEANS: It's hard not to look past the quarterfinals, with one more win each for Federer and Nadal setting up a meeting in the semifinals in what remarkably would be the first time they've met in Flushing.

There have been close calls before, with each of them making the semifinals from 2008 to 2011. In 2008, Nadal lost in the semifinals to Andy Murray, with Federer winning the title. In 2009, Federer's sixth straight US Open final appearance, Nadal lost to eventual champion del Potro in the semifinals.

The next two years, it was Federer failing to hold up his end of the bargain. He lost in five sets to Novak Djokovic in the 2010 semifinals, with Nadal winning the title, and again in 2011, dropping a two-set lead to fall to Djokovic in the semifinals with Nadal advancing to the final.

MATCH POINT: Will this be the year we finally see Federer play Nadal, a 15-time Slam title winner including twice in Queens?

Federer also can reclaim the No. 1 ranking, a title he hasn't held in almost five years, dethroning Nadal. If Federer advances to the semis and Nadal falls in the quarters, or if he beats Nadal in the semis, he'd reach No. 1 for the first time since November 2012.