Giant-killing Australian Sam Groth has claimed the scalp of world No.14 Feliciano Lopez in a three-set showdown to storm into the Mercedes Open quarter-finals in Stuttgart.

The big-serving Groth prevailed 3-6 7-6 (7-5) 7-6 (8-6) in just over two hours on Wednesday to set up a quarter-final clash with eighth seed Viktor Troicki.

Groth, who won his first-round match when opponent his Ukrainian opponent Sergiy Stakhovsky retired hurt with the Australian serving for the match, sent down 19 aces in a powerful display and saved six of the seven break points.

“Grass court is probably my favourite surface,” said the 27-year-old, who won the Manchester Challenger on grass last week.

“I had a pretty good season last year on it, it’s definitely a surface I’m comfortable on and I’ve had seven good matches now.

“I feel like I’m finally making some big steps.”

Groth will face Troicki, who is returning from a six-month ban for failing to provide a blood sample.

The 29-year-old Troicki beat 18-year-old Alexander Zverev 6-3 6-7 (6-8) 6-3.

Meanwhile, fourth seed Gael Monfils quickly found his feet on the fresh grass of Stuttgart, with the Frenchman beating Andreas Haider-Maurer 7-6 (8-6) 7-6 (7-5) thanks in part to nearly 20 aces.

The 16th-ranked Monfils ended with 18 aces and concluded both of his tiebreak sets with untouchable deliveries in the second-round victory.

Stuttgart is now the starting point for what have become three weeks of Wimbledon run-up tournaments.

The event switched from clay in July to the new date on grass directly after the end of Roland Garros.

Top seed Rafael Nadal will make his start on Thursday against former Australian Open finalist Marcos Baghdatis, while top-ranked Australia Bernard Tomic will face German veteran Tommy Haas.