Flu symptoms could not slow down Jack Sock as the unseeded American shocked top seed David Ferrer in three sets to advance to the ASB Classic final.

Sock, 23, was not feeling 100 per cent but still managed to trump the four-time champion 3-6 6-1 6-2 as his giant-killing run continued in Friday's first semi-final in Auckland.

The world number 25 rallied to beat fourth seed Kevin Anderson in the quarters and he backed that up with a performance beyond his years against ATP Tour veteran Ferrer.

After dropping the first set, Sock converted a pair of breaks in the second to force a decider and he never looked back, opening the third with a break to tally his 11th win over a top-20 player since the start of 2015.

"It was a difficult day. I woke up with some flu-like symptoms. I didn't know if I was going to play," said Sock, who will open his Australian Open campaign against a qualifier.

"I ended up coming out and playing some of my best tennis of my career. Maybe I can stay sick through Melbourne and play like this."

Robert Bautista Agut stands in the way of Sock and a second ATP title after producing a stirring comeback to see off Jo-Wilfried Tsonga.

Second seed Tsonga fell to his first defeat of the year, having blown a match point when 6-3 6-5 up in the second set, with Bautista Agut winning the subsequent tie-break thanks partly to a run of five consecutive points.

Perhaps rattled by his inability to kill the match off, Tsonga struggled badly on serve in the decider, having to fend off two break points before Bautista Agut finally broke down his resistance to reach a seventh Tour final.