Lopez was one of a 14-man group which broke away with 150 kilometres left to ride of the 187.7km stage from Carpe to Alcoy, which included seven lower-category climbs.
The 29-year-old Lopez made his decisive attack after the final climb of the day, just six kilometres from the finish.
Fellow escapees Giampaolo Caruso (Katusha), Roman Kreuziger (Liquigas) and David Moncoutie (Cofidis) could not react to the Spaniard's move, allowing Lopez to notch the biggest win of his career to date.
Kreuziger crossed the line six seconds down to take second place with Caruso and Moncoutie further behind. Moncoutie, winner of Saturday's stage eight, moved into the lead in the king of the mountains competition after amassing enough points over the second- and third-category climbs to edge ahead of overnight leader Serafin Martinez.
It was Martinez's Xacobeo Galicia team-mate Gonzalo Rabunal who showed his strength early on during the stage, crossing the first five summits in pole position and riding out alone for a period after a solo attack on the Alto de Torremanzanas.
But Rabunel hit the wall on the penultimate climb of the day and would finish the stage more than two-and-a-half minutes down on Lopez.
With the leaders at one point holding an advantage of more than eight minutes over the peloton, it looked like the red jersey of Anton was at risk.
French veteran Jean-Christophe Peraud – who won a silver medal in the Beijing Olympics for cross-country mountain biking – was virtual red jersey on the road and looked on course to overturn a 6:59 deficit.
But the Omega Pharma-Lotto rider tired towards the finish, coming home 55 seconds down on Lopez, while hard work on the front of the peloton by Anton's Euskaltel team saw the bunch cut the deficit to 7:02.
Peraud now lies in fifth place, 52 seconds off the pace of Anton, who crossed the line in Alcoy just behind Joachim Rodriguez (Katusha), who stays in second place in the overall standings. The pair share the same time, while Italy's Vincenzo Nibali (Liquigas) is two seconds back in third.
The Vuelta resumes on Tuesday September 7 with the 175.7km stage 10 from Tarragona to Vilanova i la Geltru after a rest day on Monday.