Hou -- Karjakin
I found one game in the database between these two players, and it was a game that I remember watching live three years ago. It was played during the Tata Steel Chess tournament. Hou Yifan played the French Defense and chose an obscure line of the Steinitz variation designed to immediately swap off her bad bishop. Against a player of Sergey Karjakin's caliber, this strategy was fatal. Too much time was spent in the exchange. Karjakin built up his attack and forced the Black king to remain in the middle of the board. That was enough for a lasting advantage.
Today's game was drawn after both players reached the time control. Each had two rooks and four pawns remaining.
So -- Giri
Anish Giri had White in his first meeting with Wesley So in the Corus C-Group of the 2009 Wijk aan Zee tournament. That game was agreed drawn after it was reduced to a pawn ending. Both players were elevated to the B-Group the following year. Giri won with the Petroff. Later in 2010, Giri beat So again in Biel. Their next two encounters were draws, one in the 2014 Tata Steel Masters. In last year's Tata Steel Masters, Giri won a marathon battle of 111 moves. So then scored a win against Giri in the Vugar Gashimov Memorial in April. They played six more games against one another in 2015, with So winning one of the blitz games and the others all drawn.
So won a symmetrical English that left Black's minor pieces inferior to White's.
Ding -- Adams
The only prior encounter between these two players in my database was at the 2013 Alekhine Memorial. Ding Liren played 4.f3 against Michael Adams' Nimzo-Indian Defense. Most of the pieces came off in a tactical melee, and the players agreed to a draw in a balanced endgame.
Ding Liren won an ending that featured a knight, rook, and pawns on each side. Eventually, Adams lost his knight with a stalemate trap that Ding easily saw through.
Navara -- Carlsen
According to my database, David Navara has an even score against Magnus Carlsen, but all but one of these games were played before Carlsen ascended to the top of the rating list. Navara was over 2700, and ranked above Carlsen when he beat the future World Champion with a Grunfeld Defense at Wijk aan Zee 2007. The previous year, they drew their game in the B-Group. Carlsen's win against Navara came in 2008 at the Dresden Olympiad. Two other games, most recently in 2012, ended as draws.
David Navara gave a nice interview with Yasser Seirawan where he described how he avoided the sort of strategic complications that would have favored the World Champion. In the end, Carlsen forced a draw by repetition.
Caruana -- Eljanov
All of the games between these two players have been decisive with Pavel Eljanov leading 3-2. Two games were played in the World Blitz Championship in 2010. Caruana won with White, but lost in three moves from the Black side. There must have been an illegal move or some other irregularity. The only clue that I could find in ten minutes of searching was Mark Crowther's comment (The Week in Chess, theweekinchess.com, 18 November 2010) that some games were incomplete. Perhaps the game score is incomplete due to problems with the DGT boards. Fabiano Caruana's sole classic time control win against Eljanov came against the Berlin Defense in 2011. Later that year, Eljanov opted for the Caro-Kann and won. Will Eljanov opt to test Caruana in the Berlin once again?
Caruana told Seirawan that he was worse early on in this Queen's Gambit, but his attack began to gain ground after a coupld of time wasting moves by Eljanov. The score between these two players is now even.
Wei -- Tomashevsky
If Wei Yi and Evgeny Tomashevsky have played one another prior to today, the game is not in my database. Nor do any games from the usual online database appear via a Google search.
This 23 move draw was the shortest game of the round.
Mamedyarov -- Van Wely
These two have seventeen prior encounters at my fingertips. Shakhryar Mamedyarov leads Loek Van Wely 7-7-3 (W-D-L). Two of these games were played at Wijk aan Zee.
If you showed this position to any Grandmaster and mentioned that Black has a win, he or she would find it in under two minutes. Alas, there is no such voice of assistance in a tournament game and Loek van Wely missed his chance. The game was drawn after a long ending where White was unable to convert a small advantage.
Black to move