The 23-year-old from Birmingham was not even ranked high enough for the qualifying tournament originally but withdrawals got him in and three wins put him in the main draw.
Nishikori was a huge step up but Evans took it brilliantly in his stride, winning four games in a row to take the first set and clinching a 6-4 6-4 6-2 victory in front of a packed crowd on Court 13.
Evans' run through qualifying here exceeded expectations but was not a surprise given his talent and the great form he has been showing this summer.
The 23-year-old has demonstrated a new level of commitment to tennis and is starting to achieve the results that many people always knew he could if only he knuckled down.
Prior to this tournament he reached two Challenger finals, and today's victory was his 12th in his last 14 matches, while he held match points in the other two.
Understandably it took Evans a little while to settle into the match and he was broken in the third game, but he levelled at 4-4 and then made it four games in a row to clinch the first set.
Nishikori has had an indifferent summer, but he still would not have expected the world number 179 to cause him such problems.
Evans' slice backhand was not to the Japanese player's liking at all, and another ball knifed across the net helped him break serve again to take the second set.
The start of the third was crucial, and Evans dug in to save three break points before edging closer to victory with another break.
The British number three has shone in Davis Cup over the last two years, beating players ranked much higher than himself and showing he revels on the big stage.
He has a cockiness that keeps him relaxed at such moments, and it was Nishikori who showed nerves when he stepped out to serve to stay in the match at 2-5.
Some more fine play from Evans brought him three match points and Nishikori double-faulted to hand him a quite brilliant win.
As well as a second-round match against either Bernard Tomic or Albert Ramos, the victory also guarantees Evans prize money of approximately £34,000 - quite a sum for a player who has only won £130,000 in his whole career until now.