Oscar Piastri admitted defeat in the Italian Grand Prix “hurts a lot” after he was unable to chase down Charles Leclerc.
Piastri seized the race lead from pole-sitter and team-mate Lando Norris at the second chicane on the opening lap.
The Australian then controlled the opening stanza before pitting for a set of hard compound tyres.
McLaren looked to be in the box seat until the second round of stops, when Ferrari rolled the dice and left both its cars on track.
It was an inspired decision; while Sainz was run down and fell from second to fourth, Leclerc held on to claim his second Italian Grand Prix win.
“It hurts, I’m not going to lie, it hurts a lot,” Piastri admitted.
“Did a lot of things right today. There was a lot of question marks on the strategy going into the race.
“From the position we were in with the tyres looking like they did, doing a one-stop seemed like a very risky call.
“It the end, it was right.”
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During the race, Piastri had been asked whether a one-stop was viable, the Australian making it clear he was in favour of a two-stop approach.
McLaren started both its cars on the medium compound rubber, swapping Norris onto the hards on Lap 14 and calling Piastri in from the lead on Lap 16.
Leclerc was in the lane on Lap 15, with Sainz remaining on track until Lap 19.
But while the Maranello drivers then stayed on track for the balance of the race, Norris pitted for a second time on Lap 32 as he came under pressure from Leclerc.
It was a slow stop and, as such, Norris offered no threat to Piastri, hence McLaren opted to leave him out until Lap 38.
When he emerged, he was trailing Leclerc by around 15 seconds with 15 laps remaining.
Though he held a strong tyre and pace advantage, the gap was insurmountable.
“In hindsight, yes,” Piastri admitted when asked if a one-stop was the right option.
“Everyone’s a legend on Monday after the race, or after the chequered flag.
“Today, unfortunately, we go it a bit wrong and myself being a big part of that.
“We had everything to lose from being in the lead of the race; Charles could try something a bit different – he was going to finish third either way.
“The mediums were getting destroyed and even the hards on the Red Bull at the start looked pretty dead,” he added.
“My front-left was pretty heavily grained.
“We didn’t expect it to clear back up again, which it did.”
With Piastri second and Norris third – and picking up the bonus point for fastest lap – McLaren is now just eight points behind Red Bull Racing with eight rounds remaining.