Fabian Cancellara clung on to the yellow jersey by just 0.18 seconds ahead of Lance Armstrong after Astana won a thrilling Team Time Trial stage of the Tour de France.
The powerful Astana team finished the 39km time trial stage around Montpellier in a time of 46 minutes and 29 seconds, beating the Garmin-Slipstream team, containing Bradley Wiggins and David Millar, by 18 seconds.
Crucially though, race leader Cancellara and his Saxo Bank team completed the course in a time 40 seconds slower than Astana, leaving both the Swiss rider and Armstrong dead level on time in the overall classification.
After officials checked the times carefully, Cancellara clung on to the yellow jersey by the slimmest of margins, but returning seven-time Tour champion Armstrong is hot on his rear wheel and joins him on the same time on his return to the race after four years.
Starting the time trial the last of the 20 teams, Astana knew in the closing stages that they needed to better the Saxo Bank time by over 40 seconds to ensure Armstrong made a remarkable return to wearing the yellow jersey.
However, after slowing slightly towards the finish, Astana remarkably finished exactly 40 seconds faster than Saxo Bank, and that result meant that Cancellara just about kept his yellow jersey.
With Cancellara leading and Armstrong given the same time on the overall classification, the Astana big guns are lining up behind the American, with Alberto Contador in third, while Andreas Kloden and Levi Leipheimer make up the top five.
"It makes me miss a bit the beach I was on for the last four years before all this. It's a little bit of a disappointment (to miss) but the yellow jersey is on the horizon. Astana did their maximum. That's cycling," said Armstrong, who believes Astana's powerful time trial display has ended some team's Tour hopes already.
"I think today, the Tour de France is finished for some riders," he added.
"The way the team time trial is now, you can look at the results. It's difficult, and no disrespect, but it's difficult to make up that time."
Cancellara endured a nervous wait to see if he would hold on to the lead, but it was finally confirmed that he had just held on.
"You could say it was in typical Swiss timing," he said.
"We can be proud of how we performed today. We were a little bit nervous waiting for Astana to finish - but keeping the yellow for such a small margin is just something else.
"We deserve to keep the yellow jersey and all the riders can be proud of what they achieved."
British rider Wiggins is in sixth place overall after Garmin-Slipstream's fine effort in second in the time trial, and team-mate and compatriot Millar is in tenth.
Germany's Tony Martin slipped down to eighth place from second, 52 seconds off the lead, after his Team Columbia, containing Mark Cavendish, finished in fifth place.
Stage four result:
1. Astana 46mins 29secs
2. Garmin +18"
3. Saxo Bank +40"
4. Liquigas +58"
5. Columbia +59"
6. Katusha +1:23"
7. Caisse d'Epargne +1:29"
8. Cervelo +1:37"
9. AG2R +1:48"
10. Euskaltel +2:09"
1. Fabian Cancellara (Switzerland/Saxo Bank ) 10hrs 38mins 7secs
2. Lance Armstrong (U.S/Astana ) Same Time
3. Alberto Contador (Spain/Astana ) +19"
4. Andreas Kloeden (Germany/Astana ) +23"
5. Levi Leipheimer (U.S/Astana ) +31"