BMC Racing's Porte was the undoubted favourite to win the race's crucial and most iconic stage, and in truth no other rider liked like they could dethrone him as the king of Willunga Hill, winning by eight seconds from Impey (Mitchelton-Scott) and 10 seconds ahead of third-placed Tom-Jelte Slagter (Dimension Data).
Porte attacked with 1,500m remaining, but faced an anxious wait to see if he had moved into the race's lead. Results after the stage confirmed that he had not, with Impey - who was second on GC before the stage and had 12 seconds advantage to Porte - rolling across the line eight seconds behind Porte, who collected four seconds for winning the stage.
Although level on time, Impey's second-placed finishes on stage two and four meant that he secured the leader's jersey on countback, and with just a sprint stage left, it looks as if the South African will become the race's 2018 victor.
How it unfolded
Following stage four's failure to ignite the GC battle after Peter Sagan tamed a climb to sprint to victory, the Queen stage of the race prompted a lot of excitement among the riders, and seven riders escaped the peloton once the flag was raised in McLaren Vale.
Lotto-Soudal's breakaway specialist Thomas De Gendt was joined by three of the race's most prominent figures this week, Zak Dempster and Scott Bowden from Uni SA-Australia and Nicholas Dlamini from Dimension Data. Mads Wurtz Schmidt (Katusha-Alpecin), Nuno Bico (Movistar) and Fumiyuki Beppu (Trek-Segafredo) were also among the break.
The importance of the stage meant that the break were always kept within reasonable distance to the peloton, and their lead, although it stretched out over five minutes briefly, was otherwise kept within four minutes.
The course included an ascent of Willunga Hill 22km before the finish, doubling up as the stage's King of the Mountain. When the climbing begun at 25km to the finish, Dlamini, Dempster and De Gendt distanced their breakaway companions.
De Gendt beat Dlamini to the KOM (although Dlamini secured the overall KOM jersey) and then attacked over the crest, moving into a lone lead. He powered down the descent at speeds north of 90 kmh and moved into a 55 second lead, but the Belgian was eventually reeled back in, ensuring that the climbers would have their day in South Australia.
The peloton threatened to splinter on the approach to the second ascent of the climb, but a group of around 80 riders remained intact. BMC Racing, who have showed strength in numbers this week, were absent when Porte drifted off into gravel; fortunately for the 32-year-old, he wasn't in too much trouble.
As the climb ramped upwards for the final time, Bora-Hansgrohe and LottoNL-Jumbo went to the head of the peloton, but the headwinds - which were notching around 25kmh - were preventing the prospect of early attacks.
BMC returned to their collective force to lead for a brief period, with Team Sky looking after Egan Bernal. At 2.5km to go, Sagan looked in pain and sat up, slowly drifting through the back of the lead group and relinquishing his lead in the race.
Rohan Dennis set a furious pace and Porte made his expected move at 1.5km, taking Jay McCarthy (Bora-Hansgrohe) with him. For a moment, it looked as if both Australians would ride off one another's wheel, but Porte finally distanced McCarthy.
Porte continued to dig deeper, looking to locate the required seconds needed to take the leader's jersey, and Bernal leapt off the group under the flamme rouge to try and close down Porte.
As Porte rode off into the distance passed by a deep crowd, Impey impressively passed many of the race's strongest climbers and finished eight seconds behind Porte, making sure it was he who inherited the Ochre jersey from Sagan ahead of the final stage in Adelaide.
Tour Down Under 2018, stage five: McLaren Vale to Willunga Hill, 151.5km
1. Richie Porte (Aus) BMC Racing, in 3-42-22
2. Daryl Impey (RSA) Mitchelton-Scott, at 8 secs
3. Tom-Jelte Slagter (Ned) Dimension Data, at 10 secs
4. Dries Devenyns (Bel) Quick-Step Floors
5. Egan Bernal (Col) Team Sky
6. Gorka Izagirre (Esp) Bahrain-Merida
7. Diego Ulissi (Ita) UAE-Team Emirates, all at same time
8. Robert Gesink (Ned) LottoNL-Jumbo, at 14 secs
9. Ion Izagirre (Esp) Bahrain-Merida
10. Ruben Guerreiro (Por) Trek-Segafredo, all at same time
General classification after stage five
1. Daryl Impey (RSA) Mitchelton-Scott, in 18-02-15
2. Richie Porte (Aus) BMC Racing, at same time
3. Tom-Jelte Slagter (Ned) Dimension Data, at 16 secs
4. Diego Ulissi (Ita) UAE-Team Emirates, at 20 secs
5. Dries Devenyns (Bel) Quick-Step Floors
6. Egan Bernal (Col) Team Sky
7. Gorka Izagirre (Esp) Bahrain-Merida, all at same time
8. Robert Gesink (Ned) LottoNL-Jumbo, at 24 secs
9. George Bennett (NZl) LottoNL-Jumbo
10. Ion Izagirre (Esp) Bahrain-Merida, all at same time
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