Alonso pitted at the end of lap 9 while Rosberg and Vettel left it a lap longer: it worked out well for the Ferrari which gained a place by the early stop, and then a minute later Alonso pressed his advantage to put Rosberg to the sword and assume the effective lead - although in fact Sauber's Esteban Gutiérrez was in the lead until his own first stop at the end of lap 13.
Rosberg was clearly not happy on this new set of hard tyres, and Vettel barged his way past the Mercedes on turn 6; four corners later and Felipe Massa also dismissed the German. Kimi Raikkonen had an even easier time of it now that his Lotus was on the grippier medium tyres, and he coasted past Rosberg as if on a Bank Holiday run down to the coast.
That left Alonso with a four-second lead from Vettel, Massa and Raikkonen with Rosberg now being hunted down from Webber who had benefited from his early stop to get ahead of Perez, di Resta, Daniel Ricciado and Lewis Hamilton who had now fallen to the edge of the top ten in a race going badly awry all round for Mercedes. Ricciardo pulled off a nice DRS pass on di Resta into turn 1 at the start of lap 19
Tyre wear was now becoming an issue, with Massa and Webber both in again at the end of lap 20 signalling at least a four- if not five-stop strategy. Massa's front left tyre was looking particularly worn, and the same was evident on the cars of Vettel and di Resta; Alonso was also in from the lead next time around, strongly suggesting that the hot conditions (36C on the track) were causing far more rapid degradation than the race engineers were expecting coming into the weekend and catching them out. The strategy handbook was all=but thrown out with the used Pirellis as the teams just had to react to circumstances; in Caterham's case, the contingency was having a wheel fly off Giedo van der Garde's car which forced the Dutchman to crawl his way back to pit lane on three wheels and will surely see the team slapped on the wrist for not attaching the left rear properly in the preceding pit stop.
While those around them pitted, Vettel and Raikkonen stayed out to assume the lead. Raikkonen was all over the back of the Red Bull, and at the end of lap 24 Red Bull called the race leader in for new tyres leaving the Lotus in charge; however the bid to make the current set of tyres last all the way to a three-stopper fell shorter than hoped for and Raikkonen had to head in on lap 26 after all. It seemed that no one had the upper hand and everyone as more or less in some form of crisis management and damage limitation as the midpoint of the 33-lap race neared.
Mercedes' woes were highlighted by a plaintive radio call from Hamilton who was still going backwards down the running order and was now struggling to retain 13th place: "Now I've been overtaken by a Williams!" he wailed on lap 29. When the team rubbed salt in the wound by warning him his brakes were in danger of overheating, he responded through gritted teeth: "Well, I can't drive any slower!"
As the race hit the halfway point, Alonso continued to lead by almost 14 seconds from Massa, while Vettel's tyre issues meant he lost third place to a determined Raikkonen. Webber was running a quietly impressive fifth place ahead of di Resta, Hülkenberg, Rosberg, Ricciardo and Perez after the latest round of pit stops. Most the drivers were having to get their heads down and simply slog through it, not able to entertain thoughts of gaining positions for the car ahead because of the strain it would put onto the already distressed tyres.
Lap 36 signalled the latest round of pit stops for fresh rubber during which time Raikkonen managed to pass Massa for second spot but was still well adrift of the other Ferrari. Meanwhile a collision on pit lane with Vergne's Toro Rosso left Hülkenberg's day in almost as much of a mess as his shattered front wing, and especially once the German became the latest newly-minuted recipient of a stop-go penalty for unsafe release. For once, Esteban Gutiérrez wasn't the bad boy of the Sauber team: he was out on track battling Jenson Button for seventh place, the McLaren having gone for a maximum-duration tyre conservation strategy for a three-stop afternoon along the lines as that being attempted by Raikkonen and Rosberg.
The vast majority of the field has been pulled into line on a four-stop strategy, so that as the second half of the race wore on it became clearer to see how everyone was faring relative to each other. It was increasingly clear that Alonso has a strong grip on proceedings and that Raikkonen was a solid lock for second spot. Lotus moved to solidify that position by coming in for a final set of tyres on lap 45, hoping to make the fresh set count before everyone else shed their current worn set to get on an equal footing.
Alonso was in for his final set of hard compound tyres at the end of lap 49, returning to the track with the full length of the Catalunya front stretch between him and Raikkonen. Massa dropped in behind the Lotus for the final stint and appeared to have been deployed to do his best to hold up Vettel in fourth place and take the maximum number of championship points away from the Red Bulls.