Roger Federer has a simple explanation for why he’s playing on at 34 and bidding to become the oldest grand slam champion in 45 years.

“I grew up playing against walls, cupboards, garage doors and everything. I still enjoy it,” Federer said ahead of his blockbuster US Open final with Novak Djokovic on Sunday (6.30am Monday AEST).

Sure Federer gets a kick out of playing with his four children, six-year-old twin daughters Myla Rose and Charlene Riva, and one-year-old twin boys Leo and Lenny.

But he thinks playing a final in a full house before 23,700 fans at Arthur Ashe Stadium, the biggest arena in tennis, for the first time in six years is pretty cool too.

To be playing in a record-extending 28th grand slam final when most of his challengers have come and gone is special for Federer too.

“I used to be famous for not being consistent. So for me, I never thought I could bring it in practice and in matches, week in and week out, every year,” he said.

“I never thought that was really something that I could do.

“I knew I could be surprising. I felt like I could beat anybody at any day, but I knew I had something in me that I like the big occasions, I like playing against the best and testing myself there and not shying away from that.

“But consistency was something for me that was just so far away.

“It took me a big mental step and a physical improvement to actually get to that level to play well, but I’m happy I figured it out at some stage.”

French Open champion Stan Wawrinka, who felt the full force of Federer’s attacking force in a lop-sided semi-final loss, believes his superstar countryman may well be playing the best tennis of his unparalleled career.

“It’s the best I’ve seen him play for a few years, that’s for sure,” Wawrinka said.

Federer has yet to drop a set all tournament and only been broken twice in six matches.

“I always thought I had a decent serve with variation, to be quite honest,” said the five-times winner.

“I definitely had better and worse days before, but now it seems like I’m able to just serve maybe better consistently maybe.

“I’m not sure if that’s the case, but definitely also talking about faster conditions in Cincinnati and also now here.

“I’m using conditions to my advantage, it seems like. I hope I can serve equally good come Sunday.”

The ageless champion takes a 21-20 lead over Djokovic into the final.

He also leads 3-2 in US Open meetings – including the 2007 championship match – but Djokovic has won the past two in New York, in the 2010 and 2011 semi-finals.

Both those went to five sets and the Serb knows he’ll need to bring his A-game to again deny the world No.2, as he also did in the Wimbledon final in July.

“We we all know how consistent Roger is and how good he is in the latter stages of a grand slams and any other big tournament,” said Djokovic, a nine-time major winner himself.

“I know that he’s also lately being very aggressive coming to the net, mixing up, and trying to shorten out the points. I think also he improved his speed.

“His defensive game is better than it was. Maybe healthier.

“He’s always going to perform on a high level. Rarely he drops his level. He always makes you play your best.”