If two opponent boxers pray the night before their fights who will God favor?

Prayer is not to be self-serving. This Answer applies to all sports. The best way to pray is to ask God to help prepare for the contest, and for His will to be done. If the contestant does his best, then the outcome is truly an honest sport match.

Wisdom can be given to the one who asks in faith for many endeavors, including sports: "If any of you lack wisdom, let him ask of God, that giveth to all men liberally, and upbraideth not; and it shall be given him. But let him ask in faith, nothing wavering. For he that wavereth is like a wave of the sea driven with the wind and tossed. For let not that man think that he shall receive any thing of the Lord. A double minded man is unstable in all his ways." James1:5-8.

Even if one loses a contest, he could win by learning to trust God, by gaining wisdom from above, or by being spared injury through God, who can provide safety in dangerous situations.