A spectacular long-range try from St Helens old boy Wayne McDonald condemned Super League champions St Helens to a second successive defeat and gave Leeds a psychological advantage ahead of next Saturday's Powergen Challenge Cup semi-final.

The 6ft 7in prop, released by Saints coach Ian Millward halfway through a two-year contract in 2001, showed his old boss what he has missed by romping 80 metres for a glorious match-winning try five minutes from the end of a pulsating encounter.

Great Britain centre Keith Senior had earlier grabbed two first-half tries as Leeds maintained their 100% start to the season and opened up a four-point cushion over the champions.

Leeds, who were the last team to beat Saints in a League match at Knowsley Road 18 months ago, were good value for a win that takes them back over Hull to the top of the table.

St Helens, looking to bounce back from their shock defeat at Huddersfield, scored tries at the start of each half but generally struggled to overcome an impressive Rhinos defence.

It began so promisingly for Saints, with second rower Darren Smith using a three-man overlap to touch down after less than two minutes.

Sean Long's conversion made it 6-0 but Leeds' response was immediate and decisive, with Francis Cummins palming the ball back from Andrew Dunemann's high kick for Senior to force his way over for his first try.

Saints second rower John Stankevitch pulled off a try-saving tackle on the diminutive Rob Burrow as Leeds gained the upper hand and skipper Kevin Sinfield edged his side in front with a 24th-minute penalty.

Senior then grabbed his second try, a 90-metre solo effort after scooping up a stray pass from Saints loose forward Jason Hooper following a comedy of errors in the Rhinos defence.

Referee Ian Smith had allowed play to continue after the ball had bounced off Paul Wellens' chest onto the crossbar but it was Leeds who seized the advantage.

Sinfield's third successful kick opened up an eight-point lead, although St Helens came back strongly before half-time to bring out the best in the Leeds defence.

Winger Ade Gardner dropped a pass from Paul Newlove in a threatening position and full-back Wellens had a try ruled out by video referee Steve Cross, who ruled he knocked on over the line following Long's high kick.

St Helens repeated their first-half exploits by scoring within two minutes of the re-start.

Impressive prop Darren Britt started the move, putting Hooper through a yawning gap and Sculthorpe was in support to cross for Saints' second try.

The stand-off also kicked the goal, suggesting Long had picked up an injury and the scrum-half duly went to the bench five minutes later. Smith was also withdrawn after appearing to pick up an ankle injury.

The action went from end to end until substitute Willie Poching increased the visitors lead on 52 minutes, pouncing for a try after the ball came back in the air from Saints winger Gardner.

Leeds had a chance to seal it on 63 minutes when Sinfield successfully chased a neat kick-ahead by substitute half-back Danny McGuire only to fumble the ball over the line.

The Rhinos skipper look in danger of paying the price the blunder four minutes later when centre Martin Gleeson, back after a five-match absence, regained possession from Sculthorpe's mis-cued high kick and beat two defenders to touch down.

Sculthorpe had the chance to tie the scores but his conversion attempt went wide and Sinfield calmed the visitors' nerves with a 40-metre penalty before McDonald's late winner.