Daniel Moreno (Katusha) launched a late attack on the day's final climb to secure the victory on stage four of the 2013 Vuelta a Espana on Tuesday.
Fabian Cancellara (RadioShack) placed second, with Michael Matthews (Orica-GreenEdge) in third.
Vincenzo Nibali (Astana) regained the race lead after finishing in the lead group, six seconds ahead of Horner on the stage. Nibali leads Horner overall by three seconds, with Nicolas Roche (Saxo-Tinkoff) in third at eight seconds.
The day's main escape consisted of Nicolas Edet (Cofidis), Alex Rasmussen (Garmin-Sharp), Dennis Vanendert (Lotto-Belisol), Jussi Viekkanen (FDJ) and Danilo Wyss (BMC Racing). The five riders built up a lead of around seven minutes at its peak.
Omega Pharma-QuickStep did more than its fair share to reel in the break's time advantage on the day's lumpy route. By the time the lead quintet hit the wall of the category three climb of Mirador de Ézaro - two kilometres at an average of 13 per cent, and hitting around 30 per cent in places - they had just over three minutes on the bunch.
Edet, Wyss and Vanendert quickly dropped Rasmussen and Viekkanen on the climb as they ground out their gears. The peloton also started to fall to pieces, with some riders over-geared and forced to get off and walk.
Meanwhile, Edet crested the climb solo as Wyss and Vanendert also fell back. The Frenchman managed to hang out front until 15km to go, when the largely reassembled, RadioShack-led peloton swallowed him up.
The bunch stayed together until the final two-kilometre climb to the line, when Juan Antonio Flecha (Vacansoleil) launched an attack - and just as with his similar effort on the previous day's stage, he was caught, this time by Moreno.
Moreno carried his momentum forward despite a concerted chase by Cancellara, and eased over the line with several bike lengths to spare.
The peloton split on the ascent to the line, with Horner finding himself on the wrong side of the action and losing six valuable seconds to Nibali, thereby reliquishing the red leader's jersey.
More hills tomorrow: the 174.3km fifth stage from Sober to Lago de Sanabria features two category three climbs.
Vuelta a Espana 2013, stage four: Lain to Fisterra, 189km
1. Daniel Moreno (Spa) Katusha in 4-37-47
2. Fabian Cancellara (Sui) RadioShack-Leopard
3. Michael Matthews (Aus) Orica-GreenEdge
4. Gianni Meersman (Bel) Omega Pharma-QuickStep
5. Bauke Mollema (Ned) Belkin
6. Edvald Boasson Hagen (Nor) Sky
7. Rinaldo Nocentini (Ita) Ag2r
8. Warren Barguil (Fra) Argos-Shimano
9. Sergio Henao (Col) Sky
10. Nicolas Roche (Irl) Saxo-Tinkoff all same time
Overall classification after stage four
1. Vincenzo Nibali (Ita) Astana in 14-15-30
2. Chris Horner (USA) RadioShack-Leopard at 3 secs
3. Nicolas Roche (Irl) Saxo-Tinkoff at 8 secs
Haimar Zubeldia (Spa) RadioShack-Leopard at 16 secs
5. Robert Kiserlovski (Crc) RadioShack-Leopard at 26 secs
6. Alejandro Valverde (Spa) Movistar at 21 secs
7. Rigoberto Uran (Col) sky at 28 secs
8. Daniel Moreno (Spa) Katusha at 31 secs
9. Roman Kreuziger (Cze) Saxo-Tinkoff at 42 secs
10. Bartosz Huzarski (Pol) NetApp-Endura at 45 secs
Daniel Moreno wins
Vincenzo Nibali regained the race lead
Vuelta a Espana 2013: Related links
Vuelta 2013 coverage index
Vuelta a Espana 2013: Stage reports
Stage three: Horner takes charge with stage win and overall lead
Stage two: Nicolas Roche attacks to win
Stage one: Astana takes opening team time trial
Vuelta a Espana 2013: Photo galleries
Stage three photo gallery
Stage two photo gallery
Stage one photo gallery
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