They now lead Haywards Heath by 14 points and should ensure promotion if they win one of their two remaining games, at home to Roffey on Saturday and at Worthing on the final day.
Sidley will be playing regional Division 3 cricket next season and will be joined by Steyning, Worthing, St James's Montefiore or Pulborough.
Runs were at a premium at the Stanley Turner ground where Lewes were put in by promotion rivals Littlehampton and bowled out for only only 109 as Jonathan Aspden top scroed with 30. Benn Challen (3-12) and Luke Vinter (3-24) did most of the damage.
But it proved enough as the visitors crashed to 63 all out as Simon Allan took 6-15, including a hat-trick, and Josh Burrows 3-26.
Haywards Heath had to battle for victory at home to Steyning after being put in and held to 213-7. Skipper Steve Kingcome made 41, George Martin 30 and Joe Mennie 28 but is was a league best down the order from Steve Goulds as he made 45.
Andy Stillwell led the Steyning challenge with 81 and with his team at 142-3 it looked as if Heath were heading for a fall. But then the last seven wickets fell for 20 when Hayden James returned to the attack to take 6-49. Spinner Simon Teasdale also played his part with 3-33.
Bexhill kept their promotion hopes alive with a 44-run win at home to St James's Montefiore, who are still not out of the relegation woods.
The hosts made 238-7 thanks mainly to Ashley Wright, brother of Sussex and England all-rounder Luke, who hit 87 as George Taylor took 3-48.
Saints fell short at 194 all out despite England Under-16 wicketkeeper Adam Davies making 63 and Kris Johnson 40. Pick of the home bowlers was Ian Ellis with 3-13.
Roffey, too, remain in the promotion shake up after wickets fell regularly at home to Worthing, who were rushed out for 81 by Aussie Sam Adams (6-34). But then a hat-trick from Andy Lutwyche (4-16) gave Roffey a few frights before they squeezed over the line by one wicket.
The relegation of Sidley to Division 3 East was confirmed when they lost their 13th game of the campaign, this time by 64 runs at Pulborough. The home side made 219-6 with Tom Crump hitting 91 and Lee Callaghan 42 not out before Sidley were bowled out for 155. Brad Holt (48) offered most resistance to Callaghan (5-35) and Matt Crump (4-28).