SEATTLE — Late Friday night, time was running out on CC Sabathia’s 37th birthday, which he celebrated by hurling five solid innings in a 5-1 win over the Mariners.

Because Sabathia’s contract expires at the end of the year, he was asked if where he might work next season had filtered into his head.

Sabathia explained he is focused on helping the Yankees get to the postseason and how the business side of baseball has a way of working itself out.

Then he told The Post, “Obviously my first choice is here.’’

At the start of spring training, it was easy to see Sabathia wearing another cap next season. He was in the final year of a contract that pays him $25 million for this season, 36 years old, and despite a solid 2016 he was 32-39 in the past four years.

Now, the Yankees have to give serious thought to bringing Sabathia back because of how he has pitched, the decline of Masahiro Tanaka, Michael Pineda lost due to Tommy John surgery, rookie Jordan Montgomery tailing off and top pitching prospect James Kaprielian also undergoing Tommy John and being lost for the year.

“Right now I am in the middle of this,’’ Sabathia said of the AL East race, in which the Yankees were tied for second with the Rays and 3 ½ games back of the first-place Red Sox entering Saturday’s game against the Mariners at Safeco Field. “I haven’t given a lot of thought to it. I have always let things take care of themselves, and it seems to have worked. Hopefully it will work again.’’

Without Sabathia, who is 9-3 with a 3.44 ERA in 16 starts, the Yankees wouldn’t be in the race. And if he hadn’t missed two-plus weeks with a strained left hamstring, they might be closer.

As always, general manager Brian Cashman didn’t delve into the future of a player while a season is underway.

“Now is not the time for that. I wouldn’t say if we were or were not,’’ the general manager said when asked if the Yankees’ brass had discussed Sabathia’s future internally. “We are in the stretch drive of the pennant race.’’

With Sabathia heading a rotation that could use a boost before the July 31 trade deadline.

“He has been great, has been leading the staff and a big leader in the clubhouse and performing very well on the field,’’ Cashman said.

With deep roots in North Jersey, Sabathia has often talked about staying there whenever the end arrives.

At his age, it is possible Sabathia will copy Andy Pettitte’s playbook late in his career, when the fellow lefty went year to year.

However it plays out, Sabathia has had an impressive second act.

Following two sensational starts against the Orioles in the 2012 ALDS, Sabathia was rocked for five earned runs and 11 hits in 3 ²/₃ innings by the Tigers in Game 4 of the ALCS in which the Yankees were swept.

From the start of 2013 until the end of the 2016 season, which was an improvement over the previous three years, Sabathia was 32-39 with a 4.54 ERA in 99 starts and developed a balky right knee that requires a brace when he pitches.

When healthy, Tanaka was the Yankees’ ace the past three years, but that is no longer the case. Pineda was too inconsistent to be a staff leader, and he has vanished from this year’s landscape. Luis Severino, an All-Star, still is navigating big league waters. Montgomery has 18 major league starts.

Two years ago Sabathia had to wait until the final days of spring training to find out if he was the fifth starter or headed to long relief. Now, he is one of the top reasons the Yankees are in the hunt to play in their first postseason series since 2012 and hoping to stay with a team that might need him more than he does them.