Eighth seed Jack Sock beat Marin Cilic to boost his chances of reaching the semifinals at the ATP Finals at the O2 Arena here.
The 25-year-old American beat fifth seed Marin Cilic of Croatia 5-7, 6-2, 7-6 (4) to move to 1-1 in Group Boris Becker on Tuesday, reports Xinhua news agency.
Sock, who lost to Roger Federer in his opening match on Sunday, also became the first American to win at the season finale since Andy Roddick 10 years ago.