Roger Federer does not know what to expect when he faces Novak Djokovic for the first time in two and a half years in Sunday's Cincinnati Masters final.

The pair last met in the 2016 Australian Open semi-final, Djokovic prevailing over four sets before going on to claim a sixth title at Melbourne Park.

Injuries for both men in the intervening period mean tennis fans have been deprived of another instalment of one of the sport's great rivalries, Djokovic edging the head-to-head record 23-22.

And Federer - seeking his eighth crown in Cincinnati, having defeated Djokovic in the final three times - is relishing another showdown with his old foe.

"I think what's nice about this, it's like fresh," Federer said after coming through his semi-final against David Goffin, who retired in the second set with shoulder problems on Saturday.

"It's not like we have played in the last few weeks and everybody knows what to expect. We know how we could look like, but we're not quite sure, the fans, you guys, us as well to be honest.

"A lot has happened since, with injuries both of us have been fighting, and we both came back strong again. So I think that's what's nice about this time around with Novak in the match.

"When they asked me in Indian Wells and Miami to judge Novak, I was like, 'It's not the real Novak.' He was just coming back, and he came back too soon. Same at the Australian Open. That one wasn't quite the 100 per cent Novak we know he can be.

"So I don't look at that kind of match like what could he be struggling with, because if you give him time, he'll fix that. Same with Stan [Wawrinka]. If he's got time, they will fix that. They will be very different players three months down the road.

"And look what happened after the French [Open]. Everything turned. He could have won Queen's, should have won Queen's, maybe. He ends up winning Wimbledon and he's back in another final. It looks like he hasn't missed any tennis at all over the last few years.

"I think it gives him a lot of confidence winning all those three-setters now, because in Indian Wells and Miami, what I saw there was a tired Novak, which was very rare to see. But that's why it wasn't real. He hadn't had enough practice yet, still coming fresh off his injury.

"So I think he's playing much better tennis now, much more solid off the baseline. I still think he's got room to improve. It will be interesting to see how he finishes the year."