Then, moments later, Cotterill sent a 25-yard free-kick just wide.
Swansea felt they should have had a penalty just before the half-hour mark when Pratley sliced open the visitors' defence and captain Sean St Ledger appeared to bundle Dyer over in the box, but referee Phil Gibbs waved away loud appeals.
It was forgotten 10 minutes before the break, though, as the Swans finally took a deserved lead.
Cotterill was brought down on the edge of the box by Youl Mawene and the Wales international picked himself up to curl a free-kick sweetly into the top right-hand corner.
Swansea should have doubled the lead within 90 seconds of the restart, when Dyer latched on to Pintado's flick-on and burst into the box before dragging his shot just wide.
But they did make it 2-0 just moments later when a Cotterill free-kick from the left was headed in at the near post by defender Williams for his third goal of the season.
Preston responded by throwing on Parkin and Ross Wallace for the ineffective Keith Treacy and Neil Mellor, with Darren Carter also coming on soon after.
For a time the visitors enjoyed more possession but continued to find themselves on the back-foot as the Swans pounced on countless stray passes.
Parkin did have the ball in the net for the visitors on 69 minutes with their only real chance but the flag went up for offside.
Swansea continued to threaten and Joe Allen saw a 25-yard drive pushed away by Lonergan before sending another effort over.
And Lonergan was called up again to fingertip Cotterill's 25-yard effort on to the post.
Swansea manager Paulo Sousa
Swansea City manager Paulo Sousa:
"We've had some good games but this is one of the best because we kept the quality more constantly during the game.
"Different opponents create different demands but it makes us improve. It was a complete performance. We played great football.
"If we keep up these performances we will be much stronger than ever.
"We want to improve all of the time and we feel plenty of confidence in ourselves.
"Our opponents tried to close all the space but we made decisions and gave them a lot of difficulties."
Ferguson rues lack of confidence
Preston North End manager Darren Ferguson:
"We were a little bit short on confidence after the other night's defeat.
"We didn't keep the ball well enough in the first half.
"It became more difficult for us after they scored the first goal.
"I can't question the lads in terms of their commitment and effort. We can't feel sorry for ourselves."
Live text and stats
Saturday, 6 February 2010
01 De Vries
08 Pratley yellow card
12 Dyer yellow card
24 Allen (Orlandi 87)
49 Cotterill (Butler 79)
18 Pintado (Kuqi 72)
02 Hart yellow card
12 St. Ledger
27 Nolan yellow card (Carter 64)
16 Treacy (Parkin 50 yellow card)
33 Mellor (Wallace 51 yellow card)
Ref: Phil GibbsAtt: 14,659
Attempts on target
Attempts off target
90:00+4:00 The match has reached full-time.
90:00+3:45 Andrea Orlandi takes a shot. Blocked by Sean St. Ledger.
90:00+1:00 Booking The referee cautions Nathan Dyer for unsporting behaviour.
90:00+0:45 Unfair challenge on Ross Wallace by Nathan Dyer results in a free kick. Paul Coutts restarts play with the free kick.
88:37 Free kick taken by Andrea Orlandi.
88:37 Booking Michael Hart receives a caution for unsporting behaviour.
88:33 Free kick awarded for an unfair challenge on Thomas Butler by Michael Hart.
87:57 The assistant referee signals for offside against Jon Parkin. Dorus De Vries takes the free kick.
87:00 Substitution Andrea Orlandi joins the action as a substitute, replacing Joe Allen.
82:00 Booking Darren Pratley is cautioned by the ref for unsporting behaviour.
81:37 Booking Jon Parkin goes into the book.
80:55 Unfair challenge on Jon Parkin by Leon Britton results in a free kick. Free kick taken by Andy Lonergan.
79:21 Headed effort from inside the area by Shefki Kuqi misses to the right of the goal.
78:26 Substitution (Swansea) makes a substitution, with Thomas Butler coming on for David Cotterill.
78:26 Unfair challenge on Darren Pratley by Danny Welbeck results in a free kick. Alan Tate takes the direct free kick.
77:39 David Cotterill takes a shot from just outaide the penalty box and strikes the post.
76:40 David Cotterill fouled by Chris Brown, the ref awards a free kick. Federico Bessone takes the direct free kick.
74:46 Foul by Michael Hart on David Cotterill, free kick awarded. Free kick taken by Federico Bessone.
74:00 Darren Pratley decides to take the corner short.
72:04 Joe Allen produces a drilled right-footed shot from deep inside the penalty box which goes wide of the left-hand post.
70:57 Substitution Gorka Pintado goes off and Shefki Kuqi comes on.
Correction - 70:25 Joe Allen has an effort at goal from 25 yards. Save by Andy Lonergan.
70:25 Shot from 25 yards by Joe Allen. Andy Lonergan makes a save.
69:45 Free kick awarded for a foul by Darren Carter on Nathan Dyer. Leon Britton restarts play with the free kick.
69:00 David Cotterill fouled by Paul Coutts, the ref awards a free kick. Joe Allen takes the direct free kick.
68:20 The offside flag is raised against Jon Parkin. Dorus De Vries restarts play with the free kick.
67:26 Danny Welbeck challenges David Cotterill unfairly and gives away a free kick. Ashley Williams takes the direct free kick.
67:12 Free kick awarded for a foul by Darren Pratley on Billy Jones. Sean St. Ledger takes the direct free kick.
65:08 Unfair challenge on Nathan Dyer by Paul Coutts results in a free kick. Free kick taken by Leon Britton.
63:02 Substitution Eddie Nolan goes off and Darren Carter comes on.
61:12 Booking Caution for Eddie Nolan.
60:14 Darren Pratley takes a shot. Save by Andy Lonergan.
59:05 Outswinging corner taken by Ross Wallace, clearance by Joe Allen.
58:42 Outswinging corner taken from the left by-line by Ross Wallace, clearance by Ashley Williams.
57:11 Ross Wallace fouled by Leon Britton, the ref awards a free kick. Ross Wallace takes the free kick.
56:29 Short corner worked by Paul Coutts.
55:53 David Cotterill challenges Michael Hart unfairly and gives away a free kick. Direct free kick taken by Ross Wallace.
53:52 Centre by Federico Bessone.
53:30 Federico Bessone takes a inswinging corner from the right by-line played to the near post.
53:03 Booking Ross Wallace shown a yellow card.
52:43 Nathan Dyer fouled by Ross Wallace, the ref awards a free kick. Free kick taken by Federico Bessone.
52:31 Foul by Eddie Nolan on Nathan Dyer, free kick awarded. Alan Tate takes the free kick.
50:21 Substitution Ross Wallace is brought on as a substitute for Neil Mellor.
50:00 Substitution Keith Treacy goes off and Jon Parkin comes on.
47:51 Assist on the goal came from David Cotterill.
47:51 GOAL - Ashley Williams:Swansea 2 - 0 Preston Ashley Williams grabs a headed goal from inside the six-yard box. Swansea 2-0 Preston.
47:17 Michael Hart concedes a free kick for a foul on David Cotterill. Free kick taken by David Cotterill.
46:31 Effort from inside the area by Nathan Dyer misses to the right of the target.
45:01 The second half begins.
45:00+4:03 The referee blows for half time.
45:00+3:31 Free kick awarded for an unfair challenge on Nathan Dyer by Keith Treacy. Alan Tate takes the free kick.
45:00+2:27 Nathan Dyer gives away a free kick for an unfair challenge on Chris Brown. Eddie Nolan takes the direct free kick.
45:00+1:23 The offside flag is raised against Chris Brown. Dorus De Vries restarts play with the free kick.
45:00+0:52 Alan Tate gives away a free kick for an unfair challenge on Danny Welbeck. Keith Treacy takes the direct free kick.
41:51 Unfair challenge on Chris Brown by Federico Bessone results in a free kick. Direct free kick taken by Andy Lonergan.
41:22 Corner taken by Federico Bessone from the right by-line played to the near post, clearance made by Youl Mawene.
41:03 Billy Jones challenges Alan Tate unfairly and gives away a free kick. Alan Tate takes the free kick.
38:56 Unfair challenge on Garry Monk by Danny Welbeck results in a free kick. Direct free kick taken by Garry Monk.
34:23 GOAL - David Cotterill:Swansea 1 - 0 Preston Free kick scored by David Cotterill. Swansea 1-0 Preston.
33:55 Youl Mawene challenges David Cotterill unfairly and gives away a free kick.
32:21 Free kick awarded for an unfair challenge on Nathan Dyer by Sean St. Ledger. Direct free kick taken by Alan Tate.
Correction - 27:30 Gorka Pintado is flagged offside by the assistant referee. Youl Mawene takes the indirect free kick.
27:30 The official flags Gorka Pintado offside. Direct free kick taken by Youl Mawene.
26:02 The offside flag is raised against Danny Welbeck. Federico Bessone takes the indirect free kick.
23:49 Youl Mawene gives away a free kick for an unfair challenge on Darren Pratley. Direct strike on goal from the free kick comes
in from David Cotterill.
23:38 David Cotterill takes a shot from 25 yards. Blocked by Keith Treacy.
21:33 The referee penalises Gorka Pintado for handball. Sean St. Ledger takes the free kick.
19:36 Inswinging corner taken from the right by-line by Federico Bessone, Darren Pratley has a header from close range and clears
16:51 Billy Jones fouled by Leon Britton, the ref awards a free kick. Keith Treacy restarts play with the free kick, Header from
deep inside the area by Chris Brown goes harmlessly over the crossbar.
16:05 Free kick awarded for a foul by Youl Mawene on Nathan Dyer. Dorus De Vries takes the direct free kick.
14:57 Shot from just outside the box by Joe Allen misses to the right of the goal.
10:49 Free kick awarded for an unfair challenge on David Cotterill by Paul Coutts. David Cotterill restarts play with the free kick.
9:31 Keith Treacy is caught offside. Federico Bessone restarts play with the free kick.
7:40 The assistant referee signals for offside against Chris Brown. Ashley Williams restarts play with the free kick.
7:00 Billy Jones concedes a free kick for a foul on Nathan Dyer. Alan Tate takes the direct free kick.
6:36 Federico Bessone fouled by Danny Welbeck, the ref awards a free kick. Free kick taken by Federico Bessone.
6:01 Michael Hart fouled by David Cotterill, the ref awards a free kick. Andy Lonergan takes the direct free kick.
4:47 Foul by Michael Hart on David Cotterill, free kick awarded. Direct free kick taken by Federico Bessone.
0:45 Corner taken by Federico Bessone from the right by-line, Garry Monk gives away a free kick for an unfair challenge on Andy
Lonergan. Andy Lonergan restarts play with the free kick.
0:00 The referee gets the game underway.
Live text and data provided by The Press Association.
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