Clint Dempsey added another goal for Seattle, which won its third straight game — and fourth in its last five.
NYCFC hasn't won since beating the New England Revolution 2-0 at home on March 15. It's had five losses and two draws since then.
Martins ran down a through ball and slotted a breakaway goal in the 23rd minute. He then beat goalkeeper Josh Saunders in the 66th minute after receiving a back-heel pass from Dempsey.
NYCFC's Mehdi Ballouchy got the equalizer in the 54th minute.
Sporting KC got its first win since April 5, while Chicago had its three-game winning streak halted.
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