Newport, RI, USA, 11 July 2014 | AFP

Australia’s Sam Groth has dethroned defending champion and fourth seed Nicolas Mahut 6-3 6-4 on Thursday to reach his first career ATP semifinal at the Hall of Fame Tennis Championships in Newport, Rhode Island.

Groth, the 26-year-old from Melbourne who is ranked 123rd in the world, blasted 10 aces and saved the only break point he faced to eliminate the Frenchman in 77 minutes.

“I’ve had a great grass-court season already. I really had nothing to lose,” Groth said.

“I went out there confident. I thought I played really well.”

Groth, who ousted US fifth seed Donald Young in the opening round, booked a Saturday semifinal date against Croatian second seed Ivo Karlovic, who ousted Israel’s Dudi Sela 7-6(3) 7-5 at the $US474,000 ($A512,000) grasscourt event.

With his run of three wins this week, Groth will jump into the top 100 in the world rankings and a spot in the US Open main draw.

“I’ve got myself in great shape,” Groth said.

“I’m not losing my mind out on court. I’m doing a lot of things better. I’ve tightened up my groundstrokes. I’m volleying better than I ever have. I’m making it tough for guys to break me. I’m becoming more prepared to be a top player.”

Groth has matched his number of ATP match triumphs from last year and the first half of this year combined.

Groth’s only two prior ATP singles wins of the season came in the opener at Brisbane. His lone ATP win last year came in the first round at Washington.

Karlovic, who blasted nine aces to dispatch Sela in 92 minutes, beat Groth 7-6(4) 6-3 in a first-round match at Delray Beach earlier this year.

In the quarterfinals on Friday, Aussie third seed Lleyton Hewitt will face American sixth seed Steve Johnson and US top seed John Isner meets seventh-seeded countryman Jack Sock.