Mark Edward did a fantastic job of covering the Biblical link between the serpent and Satan, so I will not re-hash that, but I would like to directly address the second part of the OPs question:
was John the first to link these two figures together? Or had the two already been connected in Jewish thought at the time? If not, is there any way to explain how the two became linked in John's mind?
Aside from the Biblical link detailed by Mark which would certainly connect the two in the Jewish mind, the Serpent as adversary, the underworld, Satan and Evil were not just connected in Jewish mind, but in the entirety of the Levant and Mesopotamia for a thousand years before the Exodus and up until (and beyond) Greco-Roman times. For example, the serpent Ladon was believed to have guarded the Golden Apples in the Garden of the Hesperides in Greek Mythology:
(Heracles and Ladon, Roman relief plate, late era.)
In fact, Serpent themes and their association with Evil appear in nearly all creation myths of the Middle East.
Nowhere is this association as striking or old as the myth of Apep arising from Egyptian Mythology from Sun Temple to Ra at Heliopolis.
According to this legend, It was thought that each day, Ra emerged from an egg and traveled in a boat known as the Bark across the sky and then crossed into the underworld at sunset. Each night, just before dawn as the Bark passes the mountain of Bahkhu in the underworld, Apep attacks the Bark according to Coffin Text Spell 160 (2181–2055 BCE):
I know that mountain of Bakhu upon which sky leans. ... On the east of that mountain is a serpent, 30 cubits in his length, with three cubits of his forefront being of flint. I know the name of that serpent who is on the mountain. His name is "He overthrows". Now at the time of evening he turns his eye over against Re, and there occurs a halting among the (solar) crew, a great astonishment(?) within the voyage, so that Seth bends himself against him.
Apep is also referenced in several of the pyramid texts:
Utterance 583
N. of Rē‘, the uraeus-serpent, which is on the forehead of Rē‘. †
Utterance 505
my companion is the uraeus-serpent, who comes forth from the god, the ’i‘r.t-serpent, who comes forth from Rē‘.
Utterance 276
To say: Thy act is against thee, what thou doest is against thee, O sksk-serpent, which is in his (thy) hole?, the opponent.
Utterance 284.
To say: He (serpent) whom Atum has bitten has filled the mouth of N., while he wound himself up (lit. wound a winding).
Utterance 288
To say: Hki-serpent or hkr.t-serpent, go away (with) face on the road. Eye of N., look not at him. Thou shalt not do thy will with N. Get away.
These references at the Pyramid of Unas make Apep the serpent among the oldest recorded legends in antiquity (2494-2345 BCE).
Furthermore, Apep is heavily associated with the Tree of Life. In the Sun Temple to Ra at Heliopolis, there was a Tree (the Ished Tree) which appears to have been the Tree of Life.
(Tree of Life from The Great Hypostyle Hall at Karnak from the 19th dynasty)
(A colorized drawing of the above)
This may be the same tree mentioned in Pyramid Text Utterance 519:
They give to Pharaoh Pepi II the tree of life whereof they live, that Pharaoh Pepi II may, at the same time, live thereof.
E.A. Budge's translation of The Book of the Dead records the following,
I am the Cat which fought by the Persea tree hard by in Annu, on the night when the foes of Neb-er-tcher were destroyed. What then is this? The male cat is Ra himself, and he is called Maau by reason of the speech of the god Sa [who said] concerning him: "He is like (maau) unto that which he hath made, and his name became Maau"; or (as others say), It is Shu who maketh over the possessions of Seb to Osiris. As to the fight by the Persea tree hard by, in Annu, it concerneth the children of impotent revolt when justice is wrought on them for what they have done.
Accompanying this writing are the following illustrations:
(From the Papyrus of Ani)
(From the Papyrus of Hu-nefer)
As you can see from this, the serpent was not unique to Christianity or related to evil/the adversary in just the Jewish mind, but all of the Levant. The serpent had, for many years been associated with not just evil and the Abyss/Underworld, but Also the Tree of Life (such as the one in Revelation 22:2 and in the first 3 chapters of Genesis). Thus this myth can be traced from the oldest known mythology to Greco-Roman mythology during the time at which John would have been writing Revelation and could explain how these were connected in in his mind (and all of Mesopotamia, really). It also makes it clear that he definitely was not the first to link the three.
† According to the Coffin Texts and the Bremner-Rhind papyrus, Ra's Sole Eye returned from searching for shu and tefnut to find that it had been replaced in its' absence. It became upset and turned into Apep.