No. 8 seed Hewitt battled back from a set down to deal Federer a rare loss on grass surfaces, where he was 76-1 since 2003 heading into Sunday's championship in Halle, Germany.
Federer's last loss in Halle was in the 2002 semifinals.
Hewitt, who had to stage a rally for the second straight match, defeated the world No. 2 in just under 2 1/2 hours. The Australian also had to bounce back from a dropped set in his semifinals victory over Benjamin Becker.
With the win -- his 29th career victory and first since April 2009 -- Hewitt also snapped a personal 15-match losing streak to Federer.
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