KANSAS CITY, Mo. -- Luis Castillo pitched six innings of three-hit ball, Jorge Polanco and J.P. Crawford drove in runs, and the Seattle Mariners beat the Kansas City Royals 2-0 on Thursday to climb back into a first-place tie with the Houston Astros in the AL West.

The Mariners are headed to Houston for a three-game series beginning Friday night.

Eduard Bazardo and Gabe Speier tossed an inning apiece for Seattle. Andres Muñoz stranded two in the ninth by striking out Adam Frazier on three pitches for his 36th save.

Castillo (10-8) dueled with Kansas City counterpart Stephen Kolek, who allowed two runs -- one earned -- while pitching into the eighth inning. One run came on Polanco's double and the other after an astute move by manager Dan Wilson in the eighth.

Kolek appeared to have retired the first two batters of the inning when Wilson challenged that second baseman Michael Massey set up on the grass ahead of Polanco's groundout. The challenge was successful and Polanco was awarded first base, and reliever Daniel Lynch IV eventually gave up Crawford's run-scoring double to make it 2-0.

Castillo and Kolek (5-6) were so stingy that each allowed just a pair of runners over the first four innings, and both times they were on back-to-back hits. The difference was the Royals paired together singles and stranded both runners while the Mariners got Josh Naylor's single and an RBI double by Polanco for a 1-0 lead.

Castillo only allowed one other runner, a single by Royals rookie Carter Jensen in the fifth. Kolek similarly breezed through the Seattle lineup until a two-out walk to Naylor in the seventh; he promptly struck out Polanco to end the inning.

The Mariners were leading 1-0 when Wilson made a rare but successful challenge. They wound up doubling their lead.

Castillo has allowed two earned runs total over his last three starts.

The Mariners begin their pivotal series in Houston with RHP Bryan Woo (14-7, 3.02 ERA) on the mound Friday night. Kansas City has RHP Michael Lorenzen (5-11, 4.91) up against Toronto on Friday night in their final home series of the regular season.

Mariners' rare challenge helps secure 2-0 win over Royals, tie Astros for first place in AL West