Cincinnati (USA), Aug 18 (IANS): Switzerland's Roger Federer, second seeded, pulled off a tough win against compatriot Stan Wawrinka who snatched the first set, to qualify for the semi-finals of the Cincinnati Masters 1000.

Federer, who has won the title in Cincinnati seven times, will now face Belgium's David Goffin, who pulled off a surprise win against Argentina's Juan Martin del Potro, fourth seeded, reports Efe.

Wawrinka, ranked 151st in the world, who has only defeated Federer three times in the last 24 matches they have played, took the first set in a tie-break, and even seemed likely in the second set to notch his fourth win against his fellow countryman on Friday.

However, Federer came back to take the second set, also in a tie-break, and then won the third set easily to seal a 6-7 (2), 7-6 (6), 6-2 win against Wawrinka.

Play was briefly interrupted due to rain.

The second semi-final will be between Serbian Novak Djokovic and Croatian Marin Cilic.