Two-time champion John Isner joined the exodus of seeds from the ATP Auckland Classic in New Zealand on Thursday, leaving the race for the title wide open.

Isner, the winner in 2010 and 2014, was seeded second this year but fell in the quarter-finals to fellow American Steve Johnson.

His exit robs the Australian Open warm-up tournament of another drawcard after other top seeds including four-time winner David Ferrer and defending champion Roberto Bautista Agut also departed for Melbourne early.

American world number 23 Jack Sock, the fouth seed, was the highest ranked player to make the semis, where he will face Johnson.

The main threat to Sock's chance of claiming a maiden ATP title is probably enigmatic Cypriot eighth seed Marcos Baghdatis, who faces Portugal's Joao Sousa in his semi-final.

Isner had complained about feeling rusty after his previous match and his woes continued in the 6-3, 5-7, 7-6 (7/3) loss as he committed 53 unforced errors.

The hardworking Johnson, ranked 33 in the world, carved out three break point opportunities early in the first set but each time Isner's thunderbolt serve blasted him out of trouble.

The tactic failed on the fourth occasion, giving Johnson a decisive edge that allowed him to comfortably serve out the set.

Isner clawed back to take a closely fought second set 7-5 after 47 minutes, then held off a string of break point chances to force a third set tie-break.

But the towering world number 19 went behind immediately as Johnson continued to target his serve and went on to claim a deserved victory.

"Hopefully I can continue that form and you never now what tomorrow holds," the Californian right-hander said.

Sock downed Jeremy Chardy 5-7, 6-4, 6-3, setting up an incredible 25 break points against the dogged Frenchman, whose resistance slowly crumbled after he claimed the first set.

On the other side of the draw, Baghdatis had no trouble brushing aside Jiri Vesely of the Czech Republic 6-2, 6-4. 

A fit-looking Baghdatis excelled as on-court temperatures reached 43 Celsius (109 Fahrenheit), racing around and looking ready for the energy-sapping conditions of Melbourne Park, where he reached the final in 2006.

Vesely, who won the only title of his career in Auckland two years ago, wilted in the heat as the Cypriot peppered him with winners.

Sousa also enjoyed a straight-forward victory, eliminating Dutchman Robin Haase 6-3, 6-2. He is yet to drop a set in Auckland and remains a dark horse contender for the title.

Haase, who upset Ferrer in the second round, looked listless after his giant-killing exploits and succumbed in less than an hour.

Source: AFP