A toss-up means that it is an unclear result; it could go either way.
1. The superb way in which both fencers were defending meant it would be a toss-up as to who would win.
2. The weather kept changing and it was a toss-up whether or not the plane would be able to land safely.
3. With top companies bidding for sponsorship rights, it would be a toss-up who would win the bid.