Wozniacki forced five break point opportunities in the first set alone - converting two of them - and only lost two points when her first set landed.
The second set continued in the same vein and Wozniacki looked set to record her fourth career `double bagel' (a 6-0 6-0 scoreline) but McHale fended off a match point at 0-5 and then - finally - held serve To the crowd's delight, the American broke Wozniacki in the next game and saved a total of four match points before the third seed finally sealed victory in just over 70 minutes.
Wozniacki faces Romanian Alexandra Dulgheru in the quarter finals on Thursday.
7.06pm: Kiwi hopes end in second round
Marina Erakovic has been knocked out of the ASB Classic after a straight sets defeat against Alexandra Dulgheru.
Erakovic, representing Kiwi hopes at the tournament, was unable to repeat her win from the first round and fell in straight sets against the 69th-ranked Romanian. Dulgheru claimed the first set in a tiebreak and two breaks of serve in the second proved enough to take the match 7-6 6-3.
6.30pm: Erakovic in trouble
Kiwi hope Marina Erakovic has lost the first set 7-6 (4) against Romanian world No 69 Alexandra Dulgheru. It's been a story of missed opportunities for the New Zealander, who has failed to capitalise on periods of momentum in the match.
4.26pm: Jelena Ostapenko loses her racquet
Tears, tantrums and a moment of true controversy at the ASB Classic today.
British world No 122 Naomi Broady claimed her opponent, Jelena Ostapenko should have been disqualified after the Latvian threw her racquet and it bounced off the ground and hit a ballboy in the second set tiebreak of their second round match.
The ball boy was called to the umpires' chair, and he confirmed the racquet had hit him on the shoulder. Broady was in tears as she first remonstrated with the umpire, then tournament supervisor Tony Cho.
Ostapenko was just as incredulous, claiming that the incident was an accident. Ostapenko appeared to clearly throw her racquet, though it didn't appear to be directed at the ballboy.
The WTA rules state that players can be defaulted for unsporting actions on court. Tim Henman was disqualified from a doubles match in his first year at Wimbledon in 1995, when he hit a ball in frustration at the end of a point and it struck a ball girl in the head.
There was obvious animosity between the players for the rest of the match, and continued at the end of the game, which was won by Broady 4-6 7-6 (4) 7-5. After a cursory handshake, the duo continued to snipe at each other and had to be separated by the umpire before the Latvian left the court.
3.35pm: Venus conqueror conquered
After playing the match of her life to upset Venus Williams yesterday, Russian youngster Daria Kasatkina is out of the ASB Classic, beaten 5-7 6-2 5-7 by Japan's Nao Hibino. She led late in the third set before falling to a Hibino comeback. As often happens, Kasatkina was unable to reach the levels of yesterday's match.
And on centrecourt, big serving Brit Naomi Broady also looks to be headed out of the tournament. World No122 Broady dropped the first set 6-4 to Latvian teenager Julia Ostapenko and trails 4-2 in the second. The 25-year-old Stockport native was irresistible from the service line yesterday against Ana Ivanovic but hasn't found the same accuracy or power today.
2.15pm: Another seed bites the dust
The carnage for the top players continues at the ASB Classic, as fouth seed Svetlana Kuznetsova is out of the tournament, beaten 6-2 6-3 by Austrian qualifier Tamira Paszek. Kuznetzova looked good on Monday in her opening round match but was never in the contest today, played off the court by Paszek, who is in irresistible form at the moment.
It was a remarkable effort by world No172 Paszek, who was playing her fifth match in five days and was on court for nearly three hours yesterday in her victory over another former grand slam winner Francesca Schiavone yesterday. However Paszek has pedigree in Auckland, having reached the 2008 semi finals here as a teenager.
However, Stephens - much to the relief of tournament organisers no doubt - has stopped the rot for the big names, coming back from a set down against German Carina Witthoeft to win 3-6 6-1 6-3.
On the outside court Russian Daria Kasatkina lost the first set 7-5 against Japan's Nao Hibino.
1.59pm: Ivanovic withdraws from doubles
Ana Ivanovic, along with her partner Kirsten Flipkens, has withdrawn from the ASB Classic doubles draw due to Ivanovic suffering from headaches and dizziness.
Ivanovic was the shock first round loser on Tuesday night - upset by world No122 Naomi Broady - but was scheduled to play her second round doubles match this afternoon.
But a tournament press release stated that "Ivanovic has made the decision alongside her doubles partner and coaching team to pull out of the Auckland tournament to recover and focus on getting to Sydney fit and healthy."
The Serbian World No16, who won the tournament in 2014, has vowed to return to the Queen city next year.
"I love being in Auckland for the ASB Classic and I really want to come back in 2017 and give the title another go.", said Ivanovic. "I really feel at home here in Auckland. I promise to all my fans that I will be back soon."
There will be sceptics that may question the validity of Ivanovic's illness but it is a reality of how things work on tour. It was a bonus for the tournament that she entered the doubles draw - and the Serbian wanted match practice - but her priority now is getting to Sydney, and getting her preparations back on track for the Australian Open.
The arrangement also suits Flipkens, who can focus on her singles match today and hopes to make a run with the field wide open.
"It's obviously disappointing for us to see Ana leave Auckland, but we know she will be back. It is unfortunate from time to time that injuries and illness happen but that is the nature of sport." said ASB Classic Tournament Director Karl Budge.
1.15pm: Seeds back in action
After the shock exits of Venus Williams and Ana Ivanovic yesterday, life continues to be difficult for the seeds at the ASB Classic. Fourth seed Svetlana Kuznetsova has dropped the first set 6-2 against Austrian qualifier Tamira Pascek, in a frustrating start for the 30-year-old Russian. Kuznetsova, who is a two time Grand Slam champion, is known as one of the best movers on the circuit but is being constantly outwitted by the clever angles and anticipation of Pascek.
Meanwhile, out on the grandstand court World No30 Sloane Stephens is struggling against unseeded German Carina Witthoeft. Witthoeft has taken the first set 6-3 against Stephens.
