Williams' chief technical officer Paddy Lowe has been talking about why Felipe Massa decided to retire at the end of 2017.
Massa originally left the sport at the end of last season. He was recalled by the team after Valtteri Bottas departed over the winter to join Mercedes.
He went on to enjoy a good season. The 36-year-old finished level on points with Renault's Nico Hulkenberg - but only three ahead of his own rookie team mate Lance Stroll. Even so, he had appeared keen to stay with at Grove in 2018.
With Sergey Sirotkin, Robert Kubica, Pascal Wehrlein, Daniil Kvyat and Paul di Resta all rumoured to be in the running to take over the seat next year, Massa sought some sort of definite decision from the team. Unfortunately, they weren't in a position to give him the assurance he needed.
"The selection of the drivers is a very complicated process," Lowe explained. "There are lots of different factors we're taking into account.
"With the timing of it all, Felipe needed a decision before Brazil," he told Motorsport.com. "We weren't able to.
"He was still in the running and was a strong candidate," Lowe added. "But we weren't able to make that commitment at that point.
"So we agreed that he would drop out of considerations and retire from the team."
Lowe has been one of Massa's most enthusiastic supporters this year, calling him "solid and dependable", adding that Massa had been "a tremendous support to Lance."
He insisted that the Brazilian had played a crucial role in the team's development in 2017.
"He was a great reference for [the performance of the FW40]," Lowe pointed out. "You always need a good reference somewhere in the garage.
"Felipe has provided that reference at every race actually, and that's been really valuable.
"There haven't been days where he's been lost," he added. "[He's always] been able to give a reference for Lance, and I think that's also quite remarkable."
Massa announced his decision to retire for good from Grand Prix racing at the start of November. He enjoyed an emotional last home race in Brazil, and a successful sign-off in the season finale at Abu Dhabi.
He plans to continue racing in stock car and karting events closer to home. He's also just been appointed the new head of the FIA's CIK/International Karting Commission.
Williams has postponed its decision over who will take over from Massa until January. It's believed that Sirotkin is all-but assured of taking the role.
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