* Del Potro knocked out by Djokovic
* Federer beats fellow Swiss Wawrinka (Adds Federer quotes)
By Simon Evans
MASON, Ohio, Aug 18 (Reuters) - Roger Federer and Novak Djokovic, the top two players in the world, set up a showdown for the Cincinnati Masters title after cruising to semi-final victories on Saturday.
The gap between the leading four, which also contains Rafa Nadal and Andy Murray, and the chasing pack was again evident as Djokovic beat Argentine Juan Martin del Potro 6-3 6-2 before Federer saw off fellow Swiss Stanislas Wawrinka 7-6 6-3.
World number one Federer is chasing a fifth title in Cincinnati while Djokovic is hoping to claim his first at the Ohio venue.
The Serb has lost in the final three times including last year when he retired injured in the second set against Murray and in 2009 when he was beaten by Federer.
Djokovic avenged his defeat by Del Potro in the bronze-medal match at the London Olympics earlier this month when he powered into his 22nd Masters 1000 final.
A tight first set turned in Djokovic's favour when sixth seed Del Potro, who was unable to capitalise on any of the six break points he had in the match, double-faulted to let his opponent go 4-2 ahead.
It was relatively straightforward in the second set after Djokovic, who won last week's Toronto Masters crown, took advantage of some loose returns to break for a 2-1 lead.
Federer has a 15-12 advantage in matches with Djokovic and won their last meeting in the Wimbledon semi-finals last month.
"We all know how good he is, especially in the later stages of events, and here conditions are quite fast so I think it's suited to his style," the Serb told reporters.
"We have played so many matches against each other. It's always a big challenge when you play a big rival of yours."
Federer needed a tiebreak to secure the first set after failing to convert any of his seven break-point opportunities but broke Wawrinka twice in the second.
"It's really tough. That's why he's number one - as soon as he gets the break he's playing really fast, serving better and playing aggressive. You have almost no chance," said Wawrinka.
Federer said his compatriot faded towards the end of their encounter.
"I thought he played really well today for at least one and a half sets. At the end he let go maybe a little bit after he got broken but those are just little things he might still be able to improve," he explained.
Federer, who won 75 percent of his service points, has already won titles this season at Wimbledon, Indian Wells, Madrid, Rotterdam and Dubai and will look for another psychological boost ahead of the U.S. Open later this month.
But the 17-times grand slam winner does not expect Sunday's final to have too much of an impact on the last grand slam of the year.
"I guess it will be helpful to win but then again it's not going to decide the outcome of the U.S. Open," said Federer.
"We will be on opposite sides of the draw and might not even play each other." (Editing by Tony Jimenez)