Jack Collison scored twice to give West Ham control of their Championship play-off semi-final after the first leg at Cardiff City.
The Welsh midfielder followed up his own shot to head home after good work from Ricardo Vaz Te on the left.
Collison then heaped further misery on his countrymen when his shot took a wicked deflection into the Cardiff net.
Kenny Miller missed a series of half-chances for Cardiff, while Ben Turner had a header cleared off the line.
Though Cardiff won 1-0 at Upton Park on the opening day of the season, Malky Mackay will need to rally his players to produce a much better performance in the return leg on Monday if the Bluebirds are to avoid a fourth play-off disappointment in as many seasons.
Sam Allardyce's Hammers were expected to claim one of the automatic promotion places and return to the Premier League at the first time of asking, only to be pipped by Reading and Southampton.
But United have now taken a huge step towards remedying that disappointment and are overwhelming favourites to claim a Wembley final date against Blackpool or Birmingham.
Cardiff nearly drew first blood when Peter Whittingham's shot was deflected just over the bar, then Miller drew a fine save from Robert Green in the West Ham goal.
But it was the Hammers who took the lead when Vaz Te broke the offside trap on the left before pulling the ball back for Collison, who had his initial effort parried by David Marshall but was able to nod the rebound into the net.
That setback in the ninth minute seemed to rattle the Bluebirds, who were lucky not to concede a second soon after.
First Liam Lawrence dawdled in the area, allowing Vaz Te to dispossess him but he was unable to get a shot away, then James Tomkins was allowed a free header at the back post that went just wide.
Slowly the home side regained their composure and Miller went close again, bending his shot just the wrong side of Green's left-hand post as the home side caught West Ham on the break.
But the visitors were also showing themselves adept at turning defence into attack and Turner had to be alert to deny Carlton Cole after Mark Hudson lost possession in the middle of the park.
From the resulting corner, Vaz Te flicked on at the near post and, although Turner cleared the initial danger, the ball fell perfectly for Collison to fire in for West Ham's second, although his shot took a wicked deflection off Lawrence.
The hosts had chances to reduce the deficit early in the second half, with Miller unable to control Lawrence's raking crossfield pass when through on Green's goal and Turner seeing his looping header cleared off the line by Cole.
West Ham were still creating the better opportunities, though, and Kevin Nolan should have put the tie beyond Cardiff but headed wide from 10 yards at the near post.
Miller was replaced by Rob Earnshaw, who fired an ambitious volley high and wide. Another volley, this time from Joe Mason, also skewed wide of goal as Turner continued to trouble the West Ham defence at set-pieces.
But the Hammers held firm under some intense late pressure and carry a precious two-goal advantage back to Upton Park for Monday's return.