Eliezer Mayenda scored a late winner as Sunderland beat Frank Lampard's Coventry 2-1 in the first leg of their Championship play-off semifinal on Friday.
Despite finishing seven points ahead of their opponents, Sunderland's form nosedived towards the end of the regular season, with five successive defeats and just one goal in a six-game winless streak.
But, after defending resolutely against a side they had lost 3-0 to in March, Sunderland were clinical on the counter-attack and Mayenda's 88th-minute goal gave the Black Cats a potentially crucial advantage.
Wilson Isidor had put Sunderland ahead midway through the second half but their opponents were level within 110 seconds thanks to Jack Rudoni.
Coventry will need to pick themselves up for Tuesday's second leg at the Stadium of Light as they bid for a second play-off final in three years.
The visitors opened the scoring when French forward Isidor ghosted into space and latched on to Enzo Le Fee's ball behind the Coventry defence before racing unchallenged into the area and firing across a helpless Ben Wilson.
But Sunderland conceded within two minutes of going ahead.
Milan van Ewijk raced to the byline and directed a cross for an unmarked Rudoni to head home.
The hosts pushed forward at every opportunity but Sunderland showed they were not settling for a draw as Patrick Roberts curled narrowly wide before Mayenda struck.
The 20-year-old took advantage of a wayward pass back from Van Ewijk to get to the ball first, round Wilson, who had hared out of his area, and coolly dispatched into an empty net.
"It's just halftime," Sunderland boss Regis Le Bris told Sky Sports. "I think we showed today that we were in our plan. We were fresh, we were connected and had all of our players available.
"We knew it'd be tough because Coventry are good and the second leg will be tight."
On Thursday, Sheffield United moved to within touching distance of a place in the Championship play-off final after beating Bristol City 3-0.