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SOFIA (Reuters) - Bosnian qualifier Mirza Basic, ranked 129th in the world, upset fourth seed Philipp Kohlschreiber 7-5 7-6(5) in a pulsating encounter to reach the quarter-finals of the Sofia Open on Wednesday.

The big-serving Basic was stronger on the key points than the German veteran, ranked 35, and moved brilliantly around the court during the one hour and 43 minute contest.

Andreas Seppi was an easy winner, dispatching Russia’s Mikhail Youzhny 6-3 6-4 to set up a meeting with world number 28 Gilles Muller in the second round on Thursday.

Top seed Stan Wawrinka, who is looking to play himself back into form after a long lay-off due to a knee injury, will open his campaign against Slovakia’s Martin Klizan on Thursday.

“He is a really dangerous player especially at this time of the year,” the 32-year-old three-times grand slam champion told a news conference. “It will be interesting to see how my knee will get that challenge.”