July 23 (Bloomberg) -- Lance Armstrong stayed on course for a record sixth success in the Tour de France after Juan Miguel Mercado edged Vicente Garcia Acosta to win stage 18 of 20 in cycling's most prestigious race following a long breakaway.

Six riders broke from the field soon after the start of today's undulating 166.5-kilometer (104-mile) trek to Lons-le- Saunier from Annemasse near the Swiss border. Mercado, 44th overall, was the best-placed at more than one hour behind Armstrong at the start of the day.

The 32-year-old American, bidding to become the oldest winner of the 101-year-old Tour de France for 24 years, finished in the pack, alongside his main rivals, almost 12 minutes behind the breakaway group. Barring an accident on the final two days, Armstrong is set Sunday to seal a sixth straight victory.

``I can't see anything stopping him now,'' said two-time Tour winner Laurent Fignon. ``Armstrong should even add to his stage wins in tomorrow's time trial.''

Yesterday, Armstrong had his third straight stage win, and fourth in six days, to stretch his overall lead to 4 minutes and nine seconds over second-place Ivan Basso of Italy. Andreas Kloden remained third, another 62 seconds back, with fellow German Jan Ullrich fourth, more than eight minutes off the lead. The time gaps remained unchanged after today's stage.

Tomorrow, there is a 55-kilometer individual time trial, or race against the clock, which starts and finishes in Besancon. Armstrong won seven of the past 10 time trials. The 91st Tour de France ends Sunday in Paris.

`Pretty Relaxed'

``Today was a pretty relaxed day, and we could ride tempo back in the pack,'' Armstrong told Francetelevision. ``It was good for everyone's legs.''

Armstrong was involved in an incident earlier, when he raced away from the pack to counter an attack from Filippo Simeoni, a rider who, away from the Tour, is involved in a legal wrangle with the Texan.

Simeoni and Armstrong caught the breakaway and relegated the peloton, or main pack, to more than two minutes behind. With the race leader among them, the break group knew the peloton wouldn't allow them to stay away.

``We asked Simeoni to drop back because if he stayed with us we were toast,'' Sebastien Joly, one of the six in the break, told Francetelevision. ``We knew Armstrong didn't want Simeoni to have a chance to win. It was all quite funny, really.''

Armstrong grinned when Simeoni decided to drop off the front group. When the main pack caught Armstrong and Simeoni, the U.S. Postal Service team leader spent time talking to other riders.

``I was protecting the interests of the peloton,'' Armstrong said. ``The other riders were very thankful.''

`Hang in There'

With Armstrong gone, the front group was given leeway and soon pulled more than 10 minutes clear. Mercado broke from the other five on the fifth and final little climb of the day before being rejoined by fellow Spaniard Garcia Acosta.

They finished 11 seconds ahead of their four counterparts, with the 26-year-old Mercado winning the dash for the line to record his first Tour de France stage win.

``Juan wanted to go home last night because he was ill but we told him to keep going, to hang in there,'' Richard Virenque, his teammate, told Francetelevision. ``We're delighted he won.''

Simeoni was not smiling. The 32-year-old Italian said Armstrong's tactics were ``not worthy of a great champion.''

``He doesn't need to worry about a little guy like me at the Tour de France,'' Simeoni told Francetelevision. ``Armstrong showed everybody just what kind of person he really is.''

To contact the reporter on this story: Darren Tulett in Paris at dtulett@bloomberg.net.

Last Updated: July 23, 2004 13:01 EDT