Stan Wawrinka extended his perfect record against Viktor Troicki to six games with a comfortable 7-5 6-2 victory in the St Petersburg Open quarter-finals.

The top seed eased into the last four in Russia with an assured display, his ground strokes crisp and powerful, and his serve reliable throughout.

Wawrinka - the US Open champion - made it nine wins in a row on the ATP Tour in just over an hour, taking advantage as errors crept in Troicki's game.

A tight opener looked set for a tie-break after 11 holds of serve, but Wawrinka upped the tempo at the decisive moment to move ahead. 

Two piercing forehands forced mistakes from his opponent to give the world number four three set points, he converted the second as Troicki blazed long.

The second followed a similar pattern but Wawrinka was putting Troicki under pressure, and in the latter stages it paid off as the Swiss won four successive games to wrap up victory.

He will meet Roberto Bautista-Agut in the semis after the fourth seed came from a set down to beat Joao Sousa 4-6 6-1 6-2.

Bautista-Agut converted five of his seven break opportunities in the match, turning it on its head after Sousa had edged the opening set.

Alexander Zverev's quest for a first ATP title moved a step closer as he booked a last-four berth at the expense of Mikhail Youzhny, the 19-year-old cruising to a straight-sets success.

The fifth seed's 6-2 6-2 triumph set up a clash with Tomas Berdych, the Czech dropping only four of his first service points as he dispatched Paolo Lorenzi 6-4 6-3 in 78 minutes.