Stan Wawrinka believes he is getting back to his best after reaching the Sofia Open semi-finals with a 6-1 7-6 (7-3) win against Viktor Troicki on Friday.

Wawrinka returned to action by beating Martin Klizan on Thursday after a knee injury contributed to his early exit from the Australian Open at the hands of Tennys Sandgren.

The Swiss had to come from behind to win his opener but made a strong start to his last-eight clash and fought back from 3-0 down in the second-set tie-break to wrap up the victory in straight sets.

"I think my level is getting better," Wawrinka said. "I'm serving and moving better and it's a great win for me. I'm doing everything to keep winning, that's for sure. I'm enjoying my time, I'm fighting on the court and that's the most important."

Wawrinka will face Mirza Basic for a place in the final following the qualifier's 6-4 4-6 6-3 win against Maximilian Marterer.

Marius Copil and Jozef Kovalik contest the other semi after seeing off Gilles Muller and Marcos Baghdatis respectively in a pair of upsets.

Top seed David Goffin was a 6-4 6-4 victor over Karen Khachanov as he made the last four of the Open Sud de France, where Richard Gasquet awaits after a 6-4 6-2 win against Damir Dzumhur.

The other semi sees Jo-Wilfried Tsonga tackle fellow Frenchman Lucas Pouille following respective wins over Andrey Rublev and Benoit Paire.