Del Potro also finished previous year strongly and the second seed saw off Denis Shapovalov 6-2 6-4 five days before the Australian Open begins, setting up a meeting with Karen Khachanov - who beat sixth seed Pablo Cuevas 6-2 7-6 (7-4).
In what proved a sterner test than his first-up victory against the equally youthful Denis Shapovalov, del Potro dominated all of the key moments.
Taking a 3-0 lead, he served his way to victory, banging 11 aces and nailing 73 per cent of his first serves against a valiant but imprecise Khachanov.
Shapovalov beat del Potro when the pair first met in Montreal last year, but the 18-year-old made too many errors Wednesday to repeat that performance.
Rejuvenated after a tough 2017 campaign, the 35-year-old Ferrer on Thursday afternoon dismantled Korean young gun Hyeon Chung 6-3 6-2.
But both times Del Potro was able to rely on his running forehand to save himself and at 4-4 he broke the Spaniard before successfully serving for the match.
Sock produced a stunning surge to qualify for the ATP World Tour Finals previous year, but the American's first match of 2018 did not go to plan.
"Tomorrow I'll have to run even more against David but it's a good challenge for me, to see how my body is before an important event like the Australian Open".
"I tried to focus during my service game and just do what I have to do".
Another former champion, fifth-seeded Roberto Bautista Agut, will face Robin Haase of the Netherlands in the other semifinal.
Juan Martin del Potro reached his third final in his last four tournaments when he beat David Ferrer in straight sets to set up an Auckland Classic decider against Robert Bautista Agut today.
"Bautista Agut is a risky player, he has experience in finals and he beat me in the past (at the 2014 Australian Open), so it's going to be hard".
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