Top seed Juan Martin del Potro has overcome a sluggish start to his season to safely progress to the quarter-finals of the Sydney International.

Playing his first match of the year, del Potro rallied from a set down to beat Frenchman Nicolas Mahut 1-6 6-3 6-4 in his tournament opener after receiving a first-round bye.

Argentina’s world No.5 next plays Radek Stepanek on Thursday for a spot in Friday’s semi-finals after the Czech Davis Cup hero defeated Spanish lucky loser Albert Ramos 6-2 7-6 (7-5).

Del Potro is seeded to meet Russia’s former champion Dmitry Tursunov in the last four.

Fourth-seeded Tursunov beat Czech Lukas Rosol 3-6 6-3 6-3 in his second-round match on Wednesday.

Australian wildcard Marinko Matosevic continued his breakthrough summer with a second-round upset win over third seed Andreas Seppi.

Matosevic, the world No.56, thrilled fans under grey skies at Sydney Olympic Park with a 6-3 6-4 victory over the 25th-ranked Seppi.

Matosevic had never won a main-draw match in Australia before this summer but is now through to back-to-back quarter-finals after also reaching the last eight at the Brisbane International.

The 28-year-old dominated Seppi from the outset and was particularly impressive on serve.

He didn’t face a single break point in 10 service games, crunched seven aces and broke the Italian once in each set to take the match in one hour, 11 minutes.

Matosevic lost to the great Roger Federer in Brisbane but has a much more inviting opportunity on Thursday against Ukraine qualifier Sergiy Stakhovsky, who ousted French sixth seed Julien Benneteau 6-3 6-2 in his second-round encounter.

Fellow Australian Bernard Tomic will be looking to join Matosevic in the quarter-finals later on Wednesday when he continues his title defence against Slovenian qualifier Blaz Kavcic.