Del Potro to face Czech hero in Sydney quarter-final
Juan Martin Del Potro of Argentina hits a return during his singles match against Nicolas Mahut of France at the Sydney International tennis tournament on January 8, 2014
The Argentine, playing his first match of the year, rallied from a set down to beat Frenchman Mahut 1-6, 6-3, 6-4 in his tournament opener after receiving a first-round bye.
World number five Del Potro will play Stepanek on Thursday for a place in the semi-finals after Stepanek defeated Spaniard Albert Ramos 6-2, 7-6 (7/5).
"For my first match in two months it was okay," Del Potro said. "The court and the balls are really fast and it's tough to play long rallies.
"Mahut serves really well and he played a lot of slices and volleys, so it was tough to feel the ball on the baseline.
"In the end I broke his serve in the third set, only once, and that was enough to close the match."
Del Potro said Stepanek will be a tough opponent early in the season.
"They won the Davis Cup, so he must feel confident playing on this surface. He has experience. We play many times. Every match was close," he said.
"I need to improve my game a little bit from the start of the match and then see what happens."
Stepanek became the first player to win two decisive fifth rubbers in two consecutive Davis Cup finals against Serbia in Belgrade in November.
Del Potro is seeded to meet former champion Dmitry Tursunov in the last four after the Russian fourth seed eliminated Czech Lukas Rosol 3-6, 6-3, 6-3 in his second-round match on Wednesday.
Australian wildcard Marinko Matosevic upset Italian third seed Andreas Seppi, 6-3, 6-4 to reach the last eight .
Matosevic, the world No.56, had never won a main-draw match in Australia before this new season but is now through to back-to-back quarter-finals after also reaching the last eight at last week's Brisbane International.
Matosevic, 28, dominated Seppi from the outset and was impressive on serve and didn't face a break point in 10 service games.
"I do it regularly overseas (reach quarters), but to do it in Australia feels good. And to beat someone like Andreas who is 20 in the world, he's a quality player," Matosevic said.
Matosevic, who lost to Roger Federer in Brisbane, will next face Ukraine qualifier Sergiy Stakhovsky, who ousted French sixth seed Julien Benneteau 6-3, 6-2 on Wednesday.