Rafael Nadal vs. Roger Federer: It's the match that everyone highlighted the minute the draw came out last week, and given their performances in their first matches on Monday, it's a tough match to call. You have to give the edge to Federer considering his form and confidence, but his lapses against Jo-Wilfried Tsonga, though minor, leave some room for doubt.
Meanwhile, Rafa's struggling with a stomach bug from some bad fish (no pun attempted, thank you very much), but showed flashes of his old self in his three-set win over Mardy Fish. Nadal has won every match they've played this year (two on clay and one on hardcourt) and he's 4-4 on hardcourts, but he's never beaten Federer at the World Tour Finals in three attempts. I'm tipping Federer, but I'll be shocked if it doesn't go three sets.
I'll be live-blogging the match from the O2 Arena in London, so tune back in at 3 p.m. ET to join the fun. In the meantime, enjoy some videos of the best moments in their rivalry.
Jo-Wilfried Tsonga vs. Mardy Fish: The only time these two played, things got a little heated at this year's U.S. Open, with Fish getting a bit hot under the collar and famously directing a snide "I don't speak French, dumb**s" at the umpire. It's all water under the London Bridge now, and both men put the odd incident behind them.
But that doesn't mean Fish won't be fired up to avenge the five-set loss. Both men have an all-court game and look to finish at the net, so it'll be a battle to see who can take control of the rallies and get there first. If Fish can serve well and control the nerves that derailed him against Nadal, I like his chances.