Sofia - Stefanos Tsitsipas, who stunned Roger Federer on his way to a shock appearance in the Australian Open semi-finals, made a winning return to action on Thursday when he defeated Germany's Jan-Lennard Struff 7-6 (7/5), 6-4 to reach the Sofia Open quarter-finals.
Second seed Tsitsipas hit 10 aces on the way to victory in his first match since losing to Rafael Nadal in the semi-finals in Melbourne last month.
The world number 12 will face seventh-seeded Frenchman Gael Monfils, who moved through by defeating Mikhail Kukushkin 7-5, 6-3 for a place in the last four.
"I will have to be careful," said Tsitsipas who stunned 20-time major winner Federer in the last-16 at the Australian Open.
"My opponent does unpredictable things on the court. Sometimes you don't expect things and he'd play, I'd say, with your mind.
"He has seen my game, I have seen his... I just need to be careful and be aware of what he's doing," the Greek star told www.atp.com.
Results from the ATP Sofia Open on Thursday (x denotes seeding):
Fernando Verdasco (ESP x6) bt Marius Copil (ROU) 6-2, 4-6, 6-1
Marton Fucsovics (HUN) bt Yannick Maden (GER) 7-6 (7/3), 3-6, 6-4
Martin Klizan (SVK) bt Daniel Brands (GER) 7-6 (7/5), 6-4
Daniil Medvedev (RUS x3) bt Robin Haase (NED) 5-7, 6-2, 6-2
Gael Monfils (FRA x7) bt Mikhail Kukushkin (KAZ) 7-5, 6-3
Stefanos Tsitsipas (GRE x2) bt Jan-Lennard Struff (GER) 7-6 (7/5), 6-4