The Hornets had stung Chelsea in the 69th minute when substitute Tamas Priskin raced clear to give the home side the lead.

But Anelka equalised in the 75th minute with a splendid overhead kick following a corner and put Chelsea in front with a header two minutes later.

After Peter Cech had denied Jobi McAnuff an equaliser with a fine save, the France striker finished off Watford with his 20th goal of the season in added time.

Anelka is sure to have impressed new boss Guus Hiddink, who will start work as Chelsea's manager for the rest of the season on Monday.

The Dutchman watched from the stand alongside owner Roman Abramovich at Vicarage Road with Ray Wilkins in charge of team affairs.

Chelsea's win saw them become the first team in the quarter-finals as the day's other games all ended in draws.

Christopher Samba took advantage of a mistake by Coventry goalkeeper Andy Marshall to earn Blackburn a last-gasp 2-2 draw in their clash at Ewood Park.

Samba pounced to level the scores in stoppage time after substitute Keith Treacy's speculative shot had come back off Marshall's chest.

Second-half strikes from Aron Gunnarsson and Michael Doyle had earlier turned the tie on its head after Roque Santa Cruz gave Rovers a second-minute lead.

Gunnarsson equalised just after the hour mark, crashing the ball home from long range after Daniel Fox's cross had been headed to only partial safety.

And with 14 minutes to go Doyle's deflected effort looped over goalkeeper Paul Robinson, but Samba spared Rovers' blushes.

Middlesbrough looked like putting their Barclays Premier League concerns behind them against in-form West Ham at Upton Park, but had to settle for a 1-1 draw after conceding a late leveller.

Boro went in front in the 22nd minute when a cross from the right by Gary O'Neil was missed by the Hammers defence and headed in at the far post by Stewart Downing.

But the visitors could not hold on and Herita Ilunga headed Gianfranco Zola's side level just seven minutes from time.

Hull and Sheffield United will also have to replay after Kamil Zayatte nodded an equaliser 11 minutes before half-time to earn the top-flight side a 1-1 draw at Bramall Lane.

Greg Halford had earlier outjumped Sam Ricketts at the far post to head a seventh-minute opener past Boaz Myhill from David Cotterill's ball.

In the day's lunchtime kick-off at the Liberty Stadium Swansea also drew 1-1 with Fulham, Jason Scotland's fine turn and shot cancelling out Garry Monk's unfortunate own goal which had put the visitors ahead.

The in-form Championship side deserved better than a replay, though, with left-winger Mark Gower firing a shot against the inside of the far post and seeing a golden chance well saved by Mark Schwarzer.