I consider myself to be an avid tennis fan, following all year round, even keeping track at the obscure tournaments and also at challenger(level below ATP) and futures level (level below challenger). So, I've came up with a list of players who I think will do well in 2013, on the back of a promising 2012 season or even just up and coming players that I expect to have a bit of a breakthrough. Milos Raonic - Any keen follower of tennis will have heard of Raonic, especially over the last 6 months or so and also after he beat Andy Murray at a recent tournament in Japan. 2011 was the year Raonic first broke through and he's carried on well into 2012, I can't see that being any different in 2013. He's still only 21 so although he may not be at a stage to challenge for grand slams just yet, I expect him to be a threat to the top players. Jerzy Janowicz - A name many will not have known up until a few weeks ago, some may still not know him. He recently got to the final of the Paris masters, beating Andy Murray on the way. He also reached the 3rd round of Wimbledon after coming through 3 qualifying rounds to get there, no mean feat. He's also had some success at challenger level, winning a few tournaments. A big serving no nonsense type of player who can only get better. He started the year ranked 221st, at one point was as high as 240th, but will end the year ranked around 26th which shows how well he has done. David Goffin - Another name many may not know, a young Belgian player who big things are expected of. He's had a bit of a poor end to the season after a promising start but if he can regain his early season form there's no reason why he can't kick on again next season. He gave Roger Federer a small scare at Roland Garros this year, taking the first set before Fed came back to beat him in 4 sets. He definitely has the talent to be a top player. Again to highlight his good year, he was ranked 174th at the start of it and will end the year in the worlds top 50. Jack Sock - A young American who has played mostly at challenger level this season with a bit a success but looks capable of making the step up to the main tour. Currently ranked in the high 100's, but as previously mentioned players have shown, if you're good enough then you can move up those rankings. The only bit of 'success' at ATP level he's had this year was making the 3rd round of the US Open after receiving a wildcard entry. Still young so with time on his side, 2013 may not be his breakthrough year but I'd expect a good showing from him. There's a few others I could mention without going into as much detail, Grigor Dimitrov, Ryan Harrison, Evgeny Donskoy, Uladzimir Ignatik, all could have very good seasons and break into the top 50 in the rankings. There's so many good players now that it's quite hard to break into the top of the rankings and make your mark but I believe the players mentioned should be good enough, if not next season then in the near future.
A quick update on this. Raonic won a tournament in San Jose in February but other than that hasn't had a great 1st half of the year. Last 16 of the Aussie open his only other notable performance. At the start of the year he was ranked 15th but has slipped down a place to 16th. Jerzy Janowicz has had a poor start to the year, the furthest he's reached in a tournament so far is the quarters finals, most recently at the Rome Masters. Hopefully an improvement to come from him later in the year. David Goffin hasn't kicked on this year in the way I thought he would, a lot of 1st and 2nd round exits for him so far this year. He recently took a step down to challenger level and played in a tournament in Bordeaux and reached the semi finals. Jack Sock has had an ok start to the year for being a young player trying to break through. His best moment so far is reaching the quarter finals at a tournament in Memphis, this helped him to make a big move up the rankings. He started the year ranked 152nd and is now currently ranked 119th. He can definitely get into the top 100 in the next few months. Of those mentioned in the thread it's one of the players I didn't go into detail about that has made the biggest improvement. Grigor Dimitrov has had a brilliant start to the year. He reached the final of Brisbane at the very start of the year in January, losing to Oor Andy. He's reached quarter finals and semi finals throughout the year, but his biggest win of the year so far came at the Madrid Masters 2 weeks ago, beating Novak Djokovic in 3 sets. He played brilliant in that match and if he can keep up that level of performance then big things could come from him.