Jonathan Rea (Kawasaki Racing Team) and Tom Sykes (Kawasaki Racing Team) were disputing the win in dry conditions until lap 14 of the 20-lap WSB opener in Portugal, when gentle and then heavy rains began to fall.
Under the flag-to-flag rules in WSB the riders made their decisions to come in to change to wet tyres, with Rea and Sykes again together on track near the end.
Sykes appeared to have a technical hiccup which allowed Rea to run away to a clear win, but Sykes hung on to second, with Chaz Davies (Aruba.it Racing - Ducati WSB) third, albeit 13 seconds back
Off the start Sykes got the lead and a good start from Rea on the third row put him second after two laps.
The two red Ducatis were just behind, joined by Haslam’s Aprilia.
Sykes was 1.222 seconds ahead after only three laps. Leon Camier (MV Agusta Reparto Corse) retired on lap three.
Alex Lowes (Voltcom Crescent Suzuki) went from fifth place to the gravel early on.
Rea started to claw back ground on Sykes from lap four onwards, having set the fastest lap of the race – 1’42.637 – on lap three.
The top four had made a successful getaway but behind a six rider fight was underway for fifth, headed by Davide Giugliano’s Ducati.
By half race distance Rea had had his first attempted pass on Sykes, rebuffed into the first corner.
One lap later Haslam had passed Davies for third, only for Haslam to make a mistake and let Davies past again, and then a bigger mistake to lose the tow.
Up front Rea eased away from Sykes to lead by 0.7 on lap 14, just as the first drops of rain were noted at turns 14 and 15. The white flag came out shortly afterwards. Sykes took it as an invitation to try and move closer to Rea.
With proper rain falling in all sectors, Rea came in to change tyres followed by most of his peers.
Sykes had remained out, but came in one lap later. Haslam fell in the wet but rejoined outside the top ten.
Coming back out of pitlane Sykes was just behind Rea as the lap totals reached parity again, and the race was on for the win on wet tyres.
Sykes led, Rea re-took the position, then Sykes again with two to go as behind Davies was five seconds down.
Sykes had some kind of machine problem, slapping his fairing as the race unwound, doing a 2’04 as Rea had put in a 1’58.
Rea ran out the winner by 9.3 seconds, with Sykes second, and Davies third. Giugliano and rain expert Sylvain Guintoli (Pata Honda WSB) were fourth and fifth respectively.