Former Udinese boss Guidolin was appointed as the man to turn Swansea around, alongside interim manager Alan Curtis, earlier in the day and he had a watching brief as Williams proved the unlikely match-winner with a header inside the opening half an hour.

Swansea had won just once from seven games in all competitions under Curtis after Garry Monk's departure in December but the host did not appear low on confidence as its adventurous start was rewarded by Williams's first Premier League goal in his last 66 outings.

In contrast, Watford - which signed Malaga forward Nordin Amrabat on Tuesday - lacked ideas as its losing run in the league was extended to four matches.

But Guidolin will begin work with a Swansea side back outside the bottom three ahead of his first game at Everton on Monday.

Off the back of a 4-2 defeat at home to struggling Sunderland last time out, Swansea brought in Jack Cork for Modou Barrow and, keen to impress its new coach, the host began brightly, albeit while struggling to create chances.

Watford named Craig Cathcart, Nathan Ake and Valon Behrami in its starting line-up after a 2-0 defeat at Southampton, with Quique Sanchez Flores's men taking time to grow into the game.

Odion Ighalo glanced Troy Deeney's cross wide 21 minutes in before Swansea responded with an Andre Ayew effort comfortably saved by Heurelho Gomes.

The host deservedly went ahead in the 27th minute when skipper Williams made the most of slack Watford defending to power home Ki Sung-yueng's cross from the right wing.

Williams's first goal for club or country since March 2014 gave Swansea further impetus as Ki fired over, while Jose Manuel Jurado had Watford's first effort on target on the stroke of half-time.

Watford was on the front foot following the restart after a lethargic first-half display as Miguel Britos headed Ben Watson's free-kick onto the roof of Lukasz Fabianski's net.

But the visitor struggled to fashion further clear-cut opportunities, while Cork sent an effort high and wide for Swansea as it sought to extend the lead.

Jurado continued to disappoint - linking neatly with Ighalo only to send a shot off target shortly after the hour as the pair, along with captain Deeney, were kept relatively quiet by Swansea's back line.

Ighalo saw penalty appeals waved away when he appeared to be held by Williams while Jurado sent another shot narrowly wide in the final stages before substitute Bafetimbi Gomis rattled Watford's right-hand post with a fierce drive.

But Williams's goal was enough to move Guidolin's men above Newcastle United and into 17th with a first win since beating West Brom on Boxing Day.