HOUSTON -- The Rangers have reached an agreement with shortstop Elvis Andrus on a contract extension that will keep him with the club at least through 2018, according to Major League sources.

The deal negotiated between the Rangers and agent Scott Boras is pending a physical and is expected to be officially announced later this week. The Rangers are not commenting until an official announcement is made.

Andrus is currently signed through 2014, and the extension is for eight years beyond that, through 2022. But the extension includes a clause that allows Andrus to opt out of the contract after four years.

Both sides expect Andrus to opt out after 2018 and become a free agent at the age of 30, unless something catastrophic happens to derail his career. By opting out, that would put Andrus in position for one final big free-agent contract. A source said Boras does not "do 33-year-old free agents."

Andrus is signed for this season at $4.8 million and for 2014 at $6.475 million. Along with a signing bonus, Andrus would get four years of $15 million per season through 2018. In the unlikely event Andrus does not opt out of the contract, he would get another four years and approximately $60 million, along with a vesting option for a ninth season.

The Rangers also have second baseman Ian Kinsler signed through 2017 on a five-year, $75 million contract, with a club option for 2018. Third baseman Adrian Beltre is signed through 2015 with a voidable club option for 2016 if he fails to reach 1,200 plate appearances in 2014-15 or 600 plate appearances in 2015.

That means the Rangers have their top infielders locked up for three to five years at a time when their top prospect is Jurickson Profar, another middle infielder. Profar is playing shortstop for Triple-A Round Rock this season.

The Rangers still have a high regard for Profar, but they don't have to make a decision on his future any time soon. At some point, Texas could make a trade from a position of strength or could ask somebody to switch positions.

Andrus, who doesn't turn 25 until Aug. 26, is in his fifth year as the club's starting shortstop. He is a two-time All-Star who has never been on the disabled list and has averaged 150 games a season. He played in 158 games in 2012, when he hit .286 with 85 runs scored, 31 doubles, nine triples, three home runs, 62 RBIs and 21 stolen bases. He had a .349 on-base percentage and a .378 slugging percentage. He is also considered one of the best defensive shortstops in the game, although he has yet to win a Gold Glove Award.

The highest-paid shortstops in baseball based on average annual salary of their current contracts are Derek Jeter of the Yankees ($18 million per season), Jose Reyes of the Blue Jays ($17.6 million) and Troy Tulowitzki of the Rockies ($15.7 million). Andrus' extension at approximately $15 million per season would put him fourth on the list.