The sprint finish seemed set up for a two-man showdown, but neither Arthur Vichot or Petr Vakoc were willing to kick on and the breakaway pair were overhauled by the peloton just yards from the line.
However the big move of the day came right near the start of the stage, when Team Sky were dropped by an aggressive break and were never able to regain the gap, with Chris Froome losing over 20 minutes and plummeting down the General Classification standings.
Faced with responding to repeated attacks on the first big climb of the day, Froome and his team-mates missed the big move and opted to ease off for the remainder of the stage, rolling in just a matter of seconds inside the 27:20 minute cut-off point.
The dropped time puts Froome well out of contention for the podium, although the three-time Tour de France winner will no doubt be happy overall with the form he's showed on his first meaningful run-out of the year.
Tour leader Valverde stretched his advantage over compatriot Alberto Contador (Trek Segafredo) to 53 seconds with another Spaniard, his Movistar team mate Marc Soler, one minute six seconds back.
Sunday's seventh and final stage will take the peloton over 138.7km around Barcelona.
Stage 6 Results:
1. Daryl Impey (South Africa / Orica) 4:34:14"
2. Alejandro Valverde (Spain / Movistar) ST
3. Arthur Vichot (France / FDJ)
4. Petr Vakoc (Czech Republic / Quick-Step)
5. Alessandro De Marchi (Italy / BMC Racing)
6. Nick van der Lijke (Netherlands / Roompot)
7. Dario Cataldo (Italy / Astana)
8. Lennard Kaemna (Germany / Sunweb)
9. Cyril Gautier (France / AG2R)
10. Daniel Martin (Ireland / Quick-Step)
GC Standings after Stage 6
1. Alejandro Valverde (Spain / Movistar) 22:18:35"
2. Alberto Contador (Spain / Trek) +53"
3. Marc Soler (Spain / Movistar) +1:06"
4. Adam Yates (Britain / Orica) +1:21"
5. Tejay van Garderen (U.S. / BMC Racing) +1:24"
6. Daniel Martin (Ireland / Quick-Step) +2:19"
7. Steven Kruijswijk (Netherlands / LottoNL) +2:46"
8. Carlos Verona (Spain / Orica) +2:50"
9. George Bennett (New Zealand / LottoNL) +2:51"
10. Romain Bardet (France / AG2R) +2:55"
With additional reporting from Reuters
Read the original article on Eurosport: Impey sprints to Stage 6 victory as Team Sky lose huge time