Zhang Mei was a setup, the alternate ending.
The Chinese team knew all about Nathan. They knew Zhang Mei was good at math, but not great. They planted her on the Chinese team, coached by her own uncle. They used her to get close to Nathan, learn what he knows, and then completely unbalance him. This was their strategy to win the competition.
Of course they had to remove her from the team, so the plan was that the uncle would "catch" them alone, together, in bed: the most traumatic experience they could legally give Nathan. Ultimately their strategy worked, Nathan completely crumbled during the final test.
It's a story done countless times, the undercover spy falls in love with the one they are spying on, and maybe that's why the film-makers decided not to reveal it. Or maybe because it would not be politically-correct to disparage China for cheating at the Olympiad (something they are known for doing at the Olypmics)
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