SAN DIEGO — The Cubs amped up their attempts to become contenders Tuesday night by reaching agreement with Jon Lester on a six-year, $155 million contract, outbidding the Red Sox in a mega-team vs. mega-team showdown.

Lester had eliminated the Giants earlier in the day, leaving it to just the Red Sox and Cubs.

Lester had played his first 8¹/₂ seasons with Boston before being traded on July 31 to Oakland. The Red Sox had made a series of low-ball offers to Lester in the months before the trade. That left Lester feeling disrespected. Boston’s ownership tried to make amends during this recruiting period with face-to-face apologies and big offers.

But in the end, the Cubs won out. Chicago has assembled a strong group of position prospects, but wanted a centerpiece starter to construct a rotation around. The Cubs have that now. The average on the deal is $25.83 million, the second largest to Clayton Kershaw.

Chicago president of baseball operations Theo Epstein left his general manager role in Boston under strained relationship. Now, he has scored a significant victory over his old employer.

The Red Sox have spent big this offseason on Pablo Sandoval and Hanley Ramirez, but remain without high-level starting pitching. The decision by Lester to go Chicago means Boston has to look elsewhere, and the possibility of making a trade with Philadelphia for Cole Hamels will go up significantly.