Diego Costa reached a Premier League milestone as Chelsea tightened their grip on the title with a 4-2 win over Southampton at Stamford Bridge.

Former Blues midfielder Oriol Romeu had levelled (24) for Southampton after Eden Hazard's early opener (5) but fit-again Gary Cahill struck just before the break and Costa netted twice thereafter - the first his 50th in England's top flight - as Antonio Conte's side stretched their lead at the top to seven points.

Nearest rivals Tottenham can cut the gap again with victory at Crystal Palace on Wednesday - live on Sky Sports - but after their FA Cup semi-final win over Mauricio Pochettino's side at the weekend, the Blues landed another psychological blow with just five games left.

Conte had restored Hazard and Costa to his line-up after starting them on the bench at Wembley, but a Blues backline bolstered by Cahill following illness had an early scare when David Luiz and Cesar Azpilicueta dithered with Manolo Gabbiadini lurking.

A smart set-piece saw Dusan Tadic loop a ball to Jack Stephens at the far post, his volley too tame to trouble, but Chelsea responded with a sweeping attack, Costa delaying his cut-back so Hazard could swing without breaking stride to find the bottom corner.

Southampton responded, Tadic and Cedric Soares involved in a one-touch move that Sofiane Boufal might have finished, but the hosts broke at ominous pace and Hazard came close to a second (18), leaning back when Costa deftly flicked into his path as Conte howled into his hands on the touchline.

Steven Davis saw a long-range effort deflected wide but the visitors drew level from the subsequent corner when Azpilicueta nodded backwards and Gabbiadini centred from an angle, Thibaut Courtois parrying only as far as Romeu as Chelsea's Premier League run without a clean sheet stretched to 11 games.

The home fans roared as Chelsea upped the tempo, N'Golo Kante seizing the moment with a driving run from wide right to byline, before Costa fell in the box under the legitimate attention of Davis and Nemanja Matic - urged on after his weekend thunderbolt - lashed off-target.

Boufal smashed into side-netting as the Saints' set-piece armoury continued to cause bother but Chelsea finished the half strongly and made it count when Marcos Alonso headed Kante's floated delivery back across goal and Cahill rose, applying a rudimentary finish as Costa shaped for an overhead kick.

Southampton snapped into tackles after half-time encouragement but Chelsea settled any jitters just minutes after the break when Cesc Fabregas showed for a short corner then scooped a ball with his laces that Costa muscled his way to meet, stooping to plant past Fraser Forster (54).

Claude Puel threw on Nathan Redmond in a bid to wrestle back momentum and Conte booted the bench as another corner carried danger, Gabbiadini blasting into the netting after a controlling touch made the angle too tight (70).

Fabregas, whose weighted pass for Costa had set up the opener, made way to hearty applause and though a late comeback never looked likely, Davis drilled a loose ball goalwards that, with Courtois committed, needed an intervention.

Chelsea tore forward as space opened up, Kante's chip palmed away by Forster (81), but the job here was done and satisfied home hordes took further delight in a late cameo for departing John Terry.

Costa sealed a convincing win a minute from time, sparking a stylish move with a give-and-go on the edge of the box before slotting low into the corner, and though there was still time for more action, Bertrand's injury-time header (90+4) a mere blot as Conte's men close in.

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Teams

Chelsea Courtois, Azpilicueta, Luiz, Cahill (c), Moses (Terry 86), Kante, Matic, Alonso, Fabregas (Pedro 77), Hazard (Willian 90), Costa
Subs Not Used Begovic, Ake, Batshuayi, Chalobah
Booked Kante, Fabregas
Goals Hazard 5, Cahill 45+1, Costa 53, 89

Southampton Forster, Soares, Stephens, Yoshida, Bertrand, Romeu, Davis (c), Ward-Prowse (Long 82), Tadic, Boufal (Redmond 68), Gabbiadini (Rodriguez 86)
Subs Not Used Hassan, Caceres, Hojbjerg, Clasie
Booked Romeu, Tadic
Goals Romeu 24, Bertrand 90+4

Attendance 41,168

Referee Lee Mason