Following Niemiec (Lampre-Merida) and Valverde (Movistar) in the thrilling stage was Joaquim Rodriguez (Katusha), Alberto Contador (Tinkoff-Saxo), Fabio Aru (Astana) and Chris Froome (Sky).
Despite the hand-to-hand combat among the overall contenders, the general classification moved only slightly, with Contador now leading leading Valverde by a reduced 31 seconds, while Froome lost a a little more than a handful to sit in third, tied with Rodriguez at 1min 20sec.
The Spanish trio of Contador, Valverde and Rodriguez repeatedly attacked to try to distance Froome on the final climb, but Contador lamented not making more of a dent in the Sky man's bid for a first Vuelta victory.
"It was perhaps a bit of shame to not open up the distance more on Froome because with a rider of that quality you always have to try and have as much of a gap as possible," he said.
"I think Froome is the strongest rival I have come across in my whole career. Maybe in this tour he is finding it a bit harder, but given the quality he has you always have to have the maximum respect and never count him out.
"At the same time I had to be aware of the attacks from Rodriguez and Valverde and in the end I am happy it is another day down and one less to go."
The stage began with the peloton facing a finale on the difficult 12.2km first category Lagos de Covadonga climb with its 7.2 per cent average gradient.
In the day-long break, Australia's Cameron Meyer (Orica-GreenEDGE) was in the mix for a podium place until the final three kilometres but was overrun by the general classification contenders and faded to 15th on the stage.
It took over 60km for a break to establish itself on the short 152.2km stage and when it did Meyer was among the five riders. His companions included Niemiec, John Degenkolb (Giant-Shimano), Kristof Vandewalle (Trek Factory Racing) and Francisco Aramendia (Caja Rural).
With the green points jersey on his back, Degenkolb was a key factor in the flatter sections of the stage, claiming the maximum points on both of the intermediate sprints on offer. And as Degenkolb was buffering his points lead, the break was also building a lead on the peloton that reached 11 minutes.
But as the the first category finale on Lagos de Covadonga grew closer the time gap steadily fell, and was all but halved on the first climb of the day, the category two Puerto del Torno.
With 30km left to race and his job done for the day, Degenkolb tailed off the break, leaving just four riders to resist the chasing peloton.
Movistar then drove the peloton on the wet tricky descent, at one point losing a rider over the edge on the twisting terrain.
With 10km to go and the peloton closing in quickly on the road to Lagos de Covadonga, Meyer made his move away from the break as Niemiec reacted to sit on his wheel.
As the climb began to hurt the activity increased in the peloton thinning out its ranks to the contenders and pretenders.
Ahead, Niemiec made his winning move, leaving Meyer to fend for himself as the battle was joined by the big favourites, including Contador, Valverde, Rodriguez and Froome.
With just six stages remaining before the champion is crowned in Santiago de Compostela in a week's time, one of just three remaining mountain finishes comes in Monday's 160.5km ride from San Martin del Rey Aurelio to La Farrapona.
SBS will broadcast eight live stages of the Vuelta a Espana, in addition to daily highlights.
Stage 15: 52.2km, Oviedo to Lagos de Covadonga
1 Przemyslaw Niemiec (POL) Lampre-Merida 4hr 11min 09sec
2 Alejandro Valverde (ESP) Movistar 0:00:05
3 Joaquin Rodriguez (ESP) Katusha
4 Alberto Contador (ESP) Tinkoff-Saxo 0:00:10
5 Fabio Aru (ITA) Astana 0:00:17
6 Christopher Froome (GBR) Sky
7 Daniel Martin (IRL) Garmin-Sharp 0:00:28
8 Warren Barguil (FRA) Giant-Shimano 0:00:44
9 Rigoberto UrÃ¡n (COL) Omega Pharma-QuickStep 0:01:00
10Giampaolo Caruso (ITA) Katusha 0:01:02
1 Alberto Contador (ESP) Tinkoff-Saxo 58hr 31min 35sec
2 Alejandro Valverde (ESP) Movistar 0:00:31
3 Christopher Froome (GBR) Sky 0:01:20
4 Joaquin Rodriguez (ESP) Katusha
5 Fabio Aru (ITA) Astana 0:02:22
6 Rigoberto UrÃ¡n (COL) Omega Pharma-QuickStep 0:02:57
7 Daniel Martin (IRL) Garmin-Sharp 0:04:55
8 Samuel SÃ¡nchez (ESP) BMC 0:05:02
9 Robert Gesink (NED) Belkin 0:05:11
10 Warren Barguil (FRA) Giant-Shimano 0:06:36